Unless you plant 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.
Problem is this is cheapo scape and I have white rocks and brown substrate. lol. And don't all those Iwugami use ADA brown substrate and ADA dark greyish sereyi stones?
I'll play around some more, just trying to work with what I have already though. See what else I have in boxes of stuff. lol.