About colours
The funny thing about drawing is that even though everyone always emphasises the importance of colours, they don't really tell you what the trick with them is. You learn about cold and warm colours, the colour charts and whatnot - then you start drawing and you're still crap with using them. Okay, practising has a lot to do with knowing which pencil to pick up and when but there's also a simple thing that helps a lot when you know about it. There's a reason why artist don't always use just one colour to paint a specific colour.
Cold and warm colours neutralize each other and in the process create a darker colour. Doesn't sound all that interesting to know, right? Well, it will when you start colouring in a person's face and wonder why your character seems to suffer from a severe sunburn. When you shade skin colour with blue and green, they neutralize the warm colours you use to create a particular skintone. No sunburn effect!
That's the main thing as far as I know and it applies to other things as well. Another colour good to know about is yellow, it's very useful in highlighting and gently filling in light areas. Of course, the way you use these colours and phases when you add them depends on the medium you're using.
But this is just the way I see it, someone smarter and with more skill might possibly tell you otherwise. A great book about all this and also about how to use colour pencils is Jose Parramon's Drawing with Colored Pencils. A picture sometimes says more than a thousand words and the examples in this book are easy to follow. The book has pictures of different phases and the information about each picture is under the picture - not hidden in the yakkity yak on the page. Who ever reads those anyway? I know I don't. I just look at the pictures. This book is really old (published somewhere in the 1980s) but maybe there's a newer version of it. I think his books are the most helpful drawing books I've seen. If you want to learn from books, learn from his.