For any given block in Minecraft, there are a handful of others with similar colors and even textures. Mixing such blocks together in walls, floors, and anything else – either randomly or in patches – can be a fantastic way to stop things from becoming a bit flat or monotone.
It works better in some scenarios than others, and it might take a bit of trial and error to find the right level of variation. But especially for larger builds, this is an amazing way to keep things interesting to look at.