And how else would you treat an item that has a different 2k and 4k texture in the database?

Using a shader to replace a texture color is not doing it "properly". Hand-painted textures have been common in the game development industry for quite a while for a good reason, shaders that simply replace colors for variety inevitably look procedural and similar to each other. Creating a new texture also gives the texture artist the ability to make edits that any new color scheme might need, such as seam, logo, or border color modifications. Not to mention that even if they did implement the new shader they would still need to create a new item in the database to reference the new shader, or the new integers at the very least. And how would a player choose to buy the item in each particular scheme if there was only one item on the market? You're quick to criticize, but it doesn't seem like you've thought it through very well.

The technique you describe is a valid one and something that might work for the level of detail some of these assets have but it is a bad idea to implement a completely different system just for the apparel since their existing workflow needs to use hand painted textures for all other assets.

And you're still missing the point that the market would need to have these different colors available anyways, or else how would you purchase the different color schemes?