For gardening, if you're wanting hybrids certain colors that are touching one another (the example by Camillion is the best way I've known to line them up) are more likely to produce specific colors (there are guides online you can find to figure out which colors = which colors).
For customizing buildings, I assume you mean the interior, to which it's really all about what you want. If you're going with a theme for your town maybe relate some character houses to match that. I've seen a lot of lovely towns with houses that represent hotels, or cafes, etc. I tend to make my Mayor's house always look really nice and formal, and then decorate any alternative character's homes to fit the personalities I perceive for them (which sounds a bit silly, in retrospect). For example, Odette I view as being very simple with a forest cottage. I'll probably only upgrade her home with a second floor and maybe a basement, but keep the size overall small. The inside will be very cozy and warm, like a comfortable haven in the midst of a dark forest.
My third character, Isolde, I view as being very artsy and nature focused. Her home is by the river, on the main side of town, and will be decorated with bright colors and lots of fauna-based items.
I'm currently redoing my entire town, though, so this might change.
Not sure if that answered your question, but I hope it helps!