Not really possible without big changes. The user-definable variables aren't necessarily for color in a shader file (they can only be float or percent, aka single numbers), so they each have to be defined separately for RGB if you want to specify an RGB value. They would have to add a new float4 user-variable type, along with an IDE component for editing it. I don't really see it being a worthwhile addition for the work required.
However, if while whoever is at it can expand the shader pipeline with PS3.0 and other goodies, than I support whoever wishes to take on the task.
From Davo:[quote:heb8ifet]For type we can either use percent or float. Currently these are all that are supported, but later we may include more (such as a texture!)[/quote:heb8ifet]
Texture type would be useful though.. Rojo..