Complementary colours

Placing opposite hues next to each other produces the strongest contrast available from colour alone, which is why orange and teal grading and orange text on blue backgrounds are all over thumbnails. The effect survives being shrunk, so it still works at the size a thumbnail is actually seen. Two cautions. Complementary pairs of similar brightness can shimmer unpleasantly when small, so vary lightness as well as hue. And red with green is the hardest pairing for colour-blind viewers, so check any thumbnail leaning on it in greyscale before you commit.