when doing an effect one can only declare what can be changed.
For colors, you can only state the three separare components. No way to add a color picker to an effect. Sorry, that color replacer fx up there is the closest you can go (I believe) to what you want in the current architecture.
Also: forget about palletes. Not happening, for reasons that have been explained a zillion times. Construct is not 8-bit. If you want 8-bit looks, you'll have to fake them yourself.