I see what you mean, so you have background colors and want to apply them to the foreground text... There is currently no command to perform that action, so it would have to be done cell by cell using color swapping. Yeah, obviously not going to happen, I know
I'm going to do a little update to REXPaint soon for another feature request, so I'll see what I can do for you in this regard and get back to you. I think one option that would work here (and possible have applications elsewhere?), would be to create a command that simply swaps the foreground and background colors throughout an image, so you could fill colors.xp with the solid foreground character (any color), then swap background/foreground colors, then use the copy feature to copy the entire thing, then paste it over the text image (with glyph application turned off--just the foreground color). How's that sound?