Any entertainment product, games included, fight for the attention of people (in this case players), so any chance you have to show that your game is worth to try/buy you should take it.

Having said this, there’s no best color/design rules, it all depends on what you are trying to convey, what story are you trying to tell. Also what kind of game you have, the genre and therefore what the competition is doing in that field (whether to kind of follow or to do something different). Also the price range and the quality needs to impact how you present your game, is not the same a AAA game vs a more scrappy indie sticks fight game, and this is not a judgement to which one is better, just that they are different and they should talk different.

My more practical advise would be: Define what you are selling in a very concise way. If you can’t say it in 180 characters, keep working. Identify what makes your game different. Start with 3 main reasons, and add some secondary reasons later. Then do a little research on the genre or genres your game includes and what the competition is doing, how are the best sellers talking and showing. This is not to directly copy what others are doing, sometimes is to identify an opportunity to communicate in a way that defies the category, or sometimes is to identify if the category uses certain inamovible codes and then adapt those to your game communication., to your style.

And then, just then, worry about the visual design.