That's tough. Most of the separate controllers I've seen are for B&W monochrome. Many of the actual products build the controller into the microcontroller as far as I know. IIRC the color controllers are usually module-specific so if you obtain any one color TFT module the chances of being able to find an LCD controller that matches it may be unlikely- and you'll probably need external RAM which is compatible with that controller too. So this gets to be a problem. You'd do well to start with a kit with an LCD controller built in like from **broken link removed**
I know they're expensive but quite a lot of people have been trying to pull this off and cheap/simple hasn't been achieved often.