Why do some books tell me to have a palette to mix my colours on?

It depends on how many different colours you purchase. If your range is limited, you will have to mix colours to create new ones. The problem with doing this, is that you usually do not mix enough colour to complete a painting and if you do not know exactly what you used and how much, then you have very little chance of mixing the exact colour again.

If you want to mix, then I suggest that you mix according to teaspoon measurements e.g. one teaspoon of green and three teaspoons of lemon will give a lime green. Then work out how many teaspoons it will take to fill your dye container. You can then work out the math e.g. it might take 15 teaspoons of green and 45 teaspoons of lemon.