There are no rules,but there is finding out what you like. Flooring coloration can be continuous and similar, contrasting, contrasting with pattern, totally random and so forth. One of the things you can do here on Houz is look about and see what you like and don't like. In the above pic, the diagonal boards are random variations on pine and hemlock, but the edging was done in fir as were the stairs. So it a kind of random but trimmed condition. The budget was minimal, so there are a lot of different woods involved.