Watercolor Notes:
When whites are lost from a painting you can lift (see below), use gouache (especially for highlights and small areas), or completely cover an area and restart it by using Watercolor Ground (Daniel Smith), Absorbent Ground (Golden Acrylics) or one of the Liquid Paper products. Some use white acrylic ink (I have not) and others scrape whites out with a sharp instrument.
To lift paint from a watercolor painting first try a damp brush and a paper towel, then try a 'soft' scrubbie brush and a paper towel, then a harder scrubbie brush and a paper towel, or for large areas use the magic sponge (Mr. Clean or any off brand). I try to lift more towards the end of the painting process if possible. It does change the texture of the paper.
A great dark/black mixture - Pthao Blue and Permanent Alizarin Crimson.
A good grey mixture - Burnt Sienna and Cobalt. Try mixing complimentary colors to see what other greys you can get. There are many more beauties to be mixed.
An interesting green - Viridian
To glaze - Mix a very wet pigment and apply it to either the entire painting or sections. I most often do this to change the temperature (warm or cool) of an area or because it looks really nice.
Gouache is opaque watercolor and can be used similar to watercolor (if a lot of water is used), but it is quite a different medium than watercolor in many ways. Lights can be added at the end and the paint lifts very easily. There is more of course, but this covers some main points.
To save your whites or certain areas of the painting you can use masking fluid (friskit) or rubber cement. This is applied when the paper is dry, before paint for saving whites or after some paint is applied and the paper is dry to save certain areas. You lift the masking or rubber cement with a rubber cement remover (made of crepe rubber) when you are done with all washes. Then, you often must do a little work to 're-attach' the white areas to the painting and/or soften the edges.
There are several ways to stretch 140 lb. paper. I often use a Zip Clamp, but they need to be ordered and are expensive and not always available. So, I also stretch by using either thick foam core or gator board. I wet both sides of the paper, place it on the board, and staple all the way around. When it dries I cover the stapled areas with tape (masking or painters tape).
Think - values, values, values!