Unless you plan to grow moss on the rocks, I'd not use white/light colored rocks with black/dark gravel. It doesn't look right as it would never occur in nature. The gravel/sand is always the same as the rocks in a stream/river.
This is a roundish black river stone I grew Flame moss on for a nano I ended up selling. This was about 30days old and ready for a trim to start the next rock. I like flame moss for this project, grows fast & thick.