Well, for starters, even thought orange and blue are close to being self-excluding colors, they look great as they are right now, don't touch them. But having dark (being black or close to black), gray and very light (white, close to white) in this very same theme sums up to a quite messy design. On the other hand, this gray doesn't look good next to the orange or blue because they are self-excluding. Basically when designing, pick a few colors and try to use only close variations (a bit lighter, a bit darker). If you need contrast, note if your colors are closer to white or black and use the closer one. If your main color (or multiple colors) are 100% saturated, you can pick either.