I dont have a favourite really. I kinda like bright colours where you can see the fish in it.
I did have a red cherry shrimp tank and I had black in there and it looked really good.
In my male betta tank I have red (ironically my betta clashes with it) - but I had the tank before I had him :).
Female betta tank - yellow.
Goldfish tank - Orange; I had intended on yellow but the shop only had enough of orange so I was pretty limited; however the orange fish and orange gravel look good haha.
Newt tank - greeny :)