Hmmm not sure what you mean by generic in this context. I for one only paint sci-fi to fit in with a specific world setting.
So my RDFs colours were chosen specifically to fit into the background of Kometenmelodie.
I would not paint them without first knowing what setting they were going to be used in.
Now… that is not to say they would not work in other settings but the idea of painting without a setting in mind gives me the unease.
If you brought any colour Space Marines to my games it would not work within the setting. They simply don’t belong. Others may have no worries on that count, but some would for sure.
If you have some of the Space Marines I would stick with official GW schemes. That way they will definitely fit into something… ?