Do you find the shadows you paint often look a bit dead, flat, too dark or have you been know to use BLACK!? If any of these points apply, I find a good solution is to use a mixture of Alizarin Crimson and Ultramarine. This mixture floated over any colour underpainting – provided it is dry and you do not brush to such an extent that it mixes with the underpainting – creates a wonderful effect. It’s worth a try.
If you use masking fluid, you’ll know that it does have a tendency to stick to the brush and is sometimes difficult to remove. Try dipping the brush in water that contains a little washing up liquid first – take of the excess fluid with a paper towel. Apply the masking fluid them IMMEDIATELY clean the brush in the water/washing up liquid mixture and things should be fine.
If you haven’t yet got a large piece of watercolour paper and mixed combinations of your watercolours to see what the results are, then you should set aside some time to do this. I know it’s boring but it is a very worthwhile exercise. Assuming you can simply mix red and blue together and it will produce a lovely purple / mauve is not always the case, you can just as easily end up with a sludgy brown. This is because some blues e.g. Prussian Blue has a green bias, mixed this with Vermillion which is orange bias and you have mud. Ultramarine mixed with Crimson which is blue bias makes a perfect purple. Watercolour painting is dependent on good colour mixing and you knowing what will happen when two colours are mixed.