Painting a room white can make it feel open, clean, spacious, quiet, or simple, and it’s always the go-to when it comes time for us to paint our interiors. But there’s more to white paint than you think. For example: tiny variations between white paints can give a vastly different effect on your space. Here are 10 things to know before you paint a room white.

1. Only a few white paints are actually pure white.

Above: The many shades of white; photograph by Mel Walbridge from Architects’ 10 Favorite Warm White Paints.

Many people mistakenly believe that all white paints are pretty much the same—and that are all devoid of color. Not the case: “White paints can come with bluish, reddish, yellowish, and even greenish undertones,” says architect Amy Alper in Remodeling 101: How to Choose the Perfect White Paint. To see the differences, follow the suggestion of interior designer Ellen Hamilton and hold the samples against a sheet of white paper. Suddenly you’ll notice the many differences and complexities in what, at first glance, seems like plain white.

Most white paints fall into three categories: warm-toned (not too bright or sterile), cool-toned (which add dimension), and “gallery” whites (which are the purest white paints, used often in art galleries). To get a sense of which is which, we surveyed architects for their favorites in each category; see:

2. All paints start out as white.

Above: Photograph by Matthew Williams for Remodelista, from DIY Paint: Designer Michaela Scherrer Mixes Her Own Shades of White.

Notes California-based designer Michaela Scherrer—whose whole Pasadena house is dressed in shades of white (and who dresses herself in only white): “All paint starts out as white,” she points out, “and then has tints added to it.” Scherrer mixes her own white paints, starting with a base paint—like Benjamin Moore Regal’s pastel base, Decorator’s White, Paper White, or the white base from Fine Paints of Europe—and adding universal tints, drop by drop, to add subtle hues. Read more about her methods (and how to mix your own custom white paints) in DIY: How to Mix the Perfect White Paint with LA Designer Michaela Scherrer.