A federal court has granted T-Mobile a preliminary injunction against AT&T subsidiary Aio Wireless on grounds that the low-cost carrier's color scheme infringes on T-Mobile's iconic magenta. In the decision, Federal District Court judge Lee Rosenthal writes that "T-Mobile has shown a likelihood that potential customers will be confused into thinking that Aio is affiliated or associated with T-Mobile based on the confused association between Aio’s use of its plum color and T-Mobile’s similar use of its similar magenta color." The opinion preliminarily prohibits Aio Wireless from using its plum color in advertising, marketing, and store design.

Aio Wireless is a low-cost, no-contract carrier owned and operated by AT&T. The prepaid service launched last year, but T-Mobile filed a lawsuit against the company immediately. In the complaint, T-Mobile argued that Aio's plum color scheme and similar wireless services confused customers into thinking that the low-cost carrier was associated with T-Mobile. Winning a preliminary injunction requires a strong case, as T-Mobile had to prove that it had a strong likelihood of success in the final case. Considering the likelihood that Aio was infringing — and the damages that the infringement was causing to T-Mobile — the court decided that it should stop Aio from continuing to use its shade of plum. A final ruling is yet to come, and issues such as monetary damages will be decided later.

"T-Mobile’s time, effort, and expense exerted to create and define its brand has been unfairly exploited."

Part of T-Mobile's argument was that "AT&T set up Aio to compete directly with T-Mobile," and the court agrees with that claim, saying that "the record is clear that Aio wanted to capture T-Mobile customers." Some documents unearthed during the case reveal that AT&T knew Aio's color scheme was similar to T-Mobile's. According to the opinion, a company hired by AT&T for focus group testing sent a report "highlighting that because the plum color was so similar to T-Mobile magenta, focus-group members were initially confused into thinking that the commercials were affiliated with T-Mobile."

T-Mobile has staunchly defended its trademark on its magenta color before, and in a statement, the company says that this latest ruling "validates T-Mobile's position that wireless customers identify T-Mobile with magenta and that T-Mobile's use of magenta is protected by trademark law."

One point of contention in the case was that Aio doesn't use the exact same color as T-Mobile; the former's is a bit more plum than magenta. The court had little issue with the differences in color, deciding that the color and services offered by Aio were similar enough to cause confusion. However, the preliminary injunction is limited to only one shade of plum. The decision specifically says that the injunction blocks Aio from using "large blocks or swaths of Pantone 676C and confusingly similar shades in its advertising, marketing, and store design." The court specifically says that the ruling "does not require Aio to abandon all uses of plum" — just the particular color Pantone 676C and similar shades.

Matt Macari contributed to this report.