So selecting colors for e.g. replacing 2 colors in a texture is not possible yet?
Colorizer was meant to be a final node. You can't do much with a colored texture since most filters act upon heightmaps.
You could easily let the user select values for replacement in heightmaps by using parameters, due to the fact that they're grayscale and parameters hold a single value, but it's not possible yet to let the user select a color from their palette as the replacement, because the palette-bound color picker only works coupled with the Colorizer.
Mind explaining what you have in mind, exactly?