So, let's see. You are building RPCs for your RGB image that you made previously, and when you click the Select GCPs in Display button, you get a display that has only one band showing, in black and white. Is that the problem? I'm pretty sure that that is just usual ENVI behavior in this case. ENVI just puts up the first band by default. You can always change that display to contain more bands, by selecting your RGB bands in the Available Bands List, and then clicking Load RGB with the Display set to the display number being used for the GCP collection.