I don’t think it would be because of solid colors because I have uploaded many shirts that use solid colors and there are countless examples of this in the catalog.
For my game I needed a T-short with a name “Slave”… it was a part of the story in the game… The Character supposed to find new clothing a little bit late. This item was not approved by Roblox, too. The Community Rules do not clearly state the reason about it.
Well, we all know roblox’s asset moderation system is quite bad, so there’s nothing surprising about people getting warned/banned for uploading a perfectly normal thing. Most safe way would be uploading stuff from an alt account to prevent your main from being unfairly banned.