The first thing you notice about the renovation of the second floor of the Johnson Building at the Boston Public Library’s central Copley Square facility—which officially opens with a ribbon-cutting starring Boston Mayor Marty Walsh Saturday morning—is the color.

“What was here before this renovation was what was there in 1972. The carpet was the same carpet,” says Michael Colford, director of library services, the number two job in the Boston Public Library system. “We had brown panels. We had gray granite. … There was never any color.”

Consider that: The blocky, brawny library addition designed by star New York architect Philip Johnson at the corner of Boylston and Exeter streets hadn’t been updated since it opened four decades ago. And “There was never any color.” Over years of wear and tear, the Johnson Building came to have a dreary, alienating warehouse feel—emphasized by the contrast with the library’s original 1895 building next door, the ornate National Landmark beaux-arts temple designed by architect Charles Follen McKim.

“This all started because we wanted a new children’s room,” Colford says. “Because our children’s room was really substandard for a library this size.” The new Children’s Library, including the StoryScape area (pictured here) with a throne for librarians to sit in during readings, is roughly twice the size of the old. Once library leaders began planning a better kids room, they came to the conclusion that they should improve the teen room, too. “From there, it just built and built and built. We ended up planning to renovate all the public spaces in this building. It was 40 years old and nothing had been done to it.” (Greg Cook)

The renewed second floor—the first phase of an ongoing renovation of the Johnson Building—radiates warm reds, purples, greens. It arrives as a surprise and a wonder. The redesign by the Boston architectural firm, William Rawn Associates, and being built by Consigli Construction, headquartered in Milford, offers a new Children’s Library, teen room, adult nonfiction shelves and lots of cozy places to sit. It’s sleek, colorful, mod, with lots of sunlight streaming in. The driving philosophy is to make the giant library more like a lounge or café, a cozy comfortable place. Coffee and snacks are welcome.

“We’re trying to create a space where people want to hang out and be in. Librarians are more mobile,” Colford says. “Through the whole Internet age, people now more than ever want to come together and be a community.”

And, notes library spokesperson Rosemary Lavery, “People are amazed that it’s all free.”

Colford points to another major improvement: “The restrooms are new. We’re going to have them on every floor. We used to only have them on the lower level.”

The Copley Square rejuvenation is part of renovations and new construction going on across the city’s library system: a new Mattapan Branch opened in February 2009, a renovated Brighton Branch reopened in December 2010, and a new East Boston Branch in November 2013. More branch projects are underway.

The new East Boston library branch that opened in November 2013. (Boston Public Library)

Work at the Copley Square library began in fall 2013. The renovations now opening cost $18 million, according to officials. “This is just phase one. The bulk of the project is still happening,” Colford says. A second phase—renovations to the first floor, the mezzanine and the exterior (including opening up windows to the streets and adding street trees)—is expected to be completed around the middle of 2016, Colford says. The total cost for the central library renovation is projected at $75.5 million.

Greg Cook, co-founder of WBUR's ARTery, is often found in libraries all over Massachusetts. Join him via Twitter @AestheticResear or the Facebook.

“The lion cubs, this is a direct nod to the McKim Building,” Colford says of the Children’s Library. They were designed by 42 Design Fab of Springfield. “Each one of them is sitting on or reading a book. In resting mode, they rotate through all the colors. They’re programmed so that as kids approach them or go under them, they light up in different ways.” (Greg Cook)

Lions—monuments to the Civil War—flank the main stairway in the 1895 McKim Building. (Greg Cook)

“It has this really cool sensory wall in it,” Colford says of the Children’s Library. “It’s really about tactile and visual and things to stimulate the senses. They’re really good for our youngest users and kids with autism or other learning disabilities.” The mural depicts the Boston Public Garden and the avian family from Robert McCloskey’s 1941 picture book “Make Way for Ducklings.” (Greg Cook)

“It’s supposed to be the Boston skyline, but created with books. They’re all by Boston authors or have Boston subject matter,” Colford says of these wall graphics in the Children’s Library that were created by Arrowstreet. The Boston design and architecture firm is developing the “wayfinding” signs for the entire building. (Greg Cook)

“The StoryScape story hour area is modeled after a Boston neighborhood with brownstones,” Colford says. These building facades in the Children’s Library were created by Mystic Scenic Studios of Norwood. (Greg Cook)

Gaps between shelves in the Children’s Library become tunnels to explore. (Greg Cook)

The Children’s Library “has a retractable glass wall that has a Curious George theme,” Colford says. “There’s a screen and projector and curtain for theatrical performances. There is small bleacher seating for kids. There’s a sink and wet room for crafts.” (Greg Cook)

The Exeter Street side of the Johnson Building—where the new Children’s Library is now—before the renovation. (Boston Public Library)

“In the original design, there were no windows in this building. They were added afterwards. When they were added, they were all tinted glass. They’ve been replaced with clear glass,” Colford says. Pictured here is a new seating area facing Boylston Street. “We had four main goals for the project and one of them was to connect the library with the street.” (Greg Cook)

Seating on the Blagden Street side of the 1972 Johnson Building before renovation. (Boston Public Library)

Tables along the central atrium and these “Windsor chairs were refinished and reused from the old,” Colford says. “A lot of shelving around the perimeter was refinished and reused.” (Greg Cook)

“The lamps they’re installing are our nod to Bates Hall, the big reading room in the McKim Building,” Colford says. Refurbished tables, like much of the new furnishings, they offer power and data ports. They ring the central atrium to take advantage of the natural light. Four floors of closed-to-the-public book stacks rise behind these walls. “The skylight will be repaired,” Colford says. (Greg Cook)

Bates Hall in the library’s 1895 McKim Building. (Greg Cook)

The atrium of the Johnson Building before renovation. (Boston Public Library)

“Banners are new to guide people to different collection areas,” Colford says. Digital displays are also sprinkled throughout the adult nonfiction shelves to tease the library’s special collections. “This is a nod to show people what they don’t see on the floor.” (Greg Cook)

Teen Central offers a lounge to watch movies or play video games, with screens that tilt so they can face away from each other for competitive gaming. Décor includes old street signs and an MBTA system map. There’s also a “digital maker space,” with a 3D printer and video and audio editing equipment. (Greg Cook)

“This is the original 1972 slab. We ripped the carpet off and polished the slab,” Colford says of Teen Central. “We wanted to do a more urban industrial feel.” (Greg Cook)

“They will be able to eat over here,” Colford says of Teen Central. “This is really designed for them to hang out.” (Greg Cook)

A map of the newly renovated second floor of the Johnson Building. (Greg Cook)

The Boylston Street entrance to the 1972 Johnson Building before renovation. (Boston Public Library)

New street trees are planned for the Boylston and Exeter corner of the Johnson Building as shown in this architect’s rendering of the renovation. (Boston Public Library)