Since OptiFine has a cape editor for user-selected colors, I think FO should have one too!

I don't get your question. FO doesn't host capes and the tutorial to get one is already in the wiki...

I don't get your question. FO doesn't host capes and the tutorial to get one is already in the wiki..

I meant a website where you would be able to change the colors of the text and the background color of the cape, like the MinecraftCapes' OF cape editor.

Right.
Well, I've never seen anyone with a FO cape besides myself, so I don't think there is much interest in this. Especially because users can use any image they want 😛

If you want, you could find an online image tool that can take an url and change the image's hue (colors) and write a tutorial about it in the wiki.