I kind of hate the look of rug over rug. Grey over brown does not sound very nice to me.

Yeah, well, if you must, you might look at pattern over solid. Or texture over smooth. Or wool over grass.

But gray cut pile over beige cut pile, probably not.

Sometimes it is necessary. My late mother had a lovely oriental over the wall-to-wall carpet in her LR. It took three try's before the oriental rug dealer finally sold her the right pad so the rug didn't "walk" on the carpet.

When we were living in a dorm faculty apt in Maine, the entire apt was wall-to-wall brown indoor/outdoor carpet glued to concrete. UGH! We just put rugs over as much of it as possible. I know knew what pad to use after my mother's experience. Unfortunately, I have no memory of what it was except that it was thin and kind of sticky. Think it might have been white and sort of looked like white Pelham interlining.

I wouldn't do solid over solid. I have had to have rugs over my carpet in two places now. Not ideal, but a place keeper until better flooring can be installed. DO NOT put a cheap area rug over cheap carpet, the cheap area rugs do not last, just wasting money.