Light & Colour Practice
You can never get enough of the basics! Do a red ball today, a blue ball tomorrow. Do a yellow square next week, blow up the moon next year!

...Okay, maybe not blow up the moon, but you get the point!

If your having troubles understanding colour and its relationships, all you can really do is practice and study. Colour is a big monster to understand and has many things that effect how it works. Lighting and the colour of that light, reflections, colour combinations, surface texture, they all effect how we see colour.