I agree with every plant author I have so far read in recommending fine-grain gravel (grains 1-2 mm) as the best for planted aquaria. This is not to say other substrates won't work; but as you asked about gravel, there it is.
As for colour, the darker is best both for the plants and in most cases the fish, assuming we generally maintain "forest fish" in planted aquaria; fish like tetras, rasbora, catfish, loaches, dwarf cichlids, etc. all prefer dark substrates because it gives them security. This is general, there are a few exceptions, but darker substrates tend to work better in my opinion.
I would avoid the bags of special gravel, they are very expensive. Most fish stores carry bulk gravel, usually they have the buff/brown "natural" kind and sometimes if lucky the darker black/gray gravel. I have always bought in bulk, much less expensive and it is good inert gravel. If you buy packaged, make sure it is inert, i.e., it will not raise hardness and pH due to being calcareous in composition. Gravels intended for marine tanks, livebearers or rift lake cichlids will often do this; not what you want in a freshwater planted aquarium with soft acidic water fish (and plants).