This is a great tip. Make sure you use a large water container to rinse out your brushes. Often times I see people trying to paint an 11 x 15 inch (1/4 sheet) watercolor painting and they use a tiny cup of water to clean their brushes. Think big! The bigger your water container, the longer the water will stay clean and the fresher your colors will be. Guess what, dirty water will transfer to your painting.
Make sure you have a container that will hold at least one quart or one liter of water. I like to paint using at least two water containers and sometimes three. You don't need a fancy water container. Think green, recycle. Wash and reuse a food container. I have a bunch of organic yogurt containers which I bring to my classes to give to students who bring in a teeny tiny cup.
Shown here is exactly what is on my painting table this morning. One Tupperware container which was forgotten at my house after a party. I still do not know who it originally belonged to, but it makes a great water container for painting. One lingonberry container (fabulous swedish berries) and one quart sized organic yogurt container.
I always keep one filled with clean clear water, perfect for blending away and softening an edge or wetting the paper. The other two end up with different stages of dirty water. As you paint, if the water becomes real dirty, change it. Dump it out and start with fresh. You don't want to be able to paint color on your paper with your water because most of the time it is an ugly grey-ish brown.