Things to think about when planning a background to a watercolour painting
Backgrounds needn't be complicated but they do need to compliment or enhance the focal point. Nothing in the background should be more prominent than the focal point so it's something that needs to be thought about at the beginning of a painting, not half way through or worse, at the end.
Below are some things to think about.
After identifying the highlights or white areas of the painting the next step would be to decide whether the background will be lighter or darker in tone than the focal point. It may be both, this is called counter-change and can be used to great effect in a painting. The mood of the painting will be affected by the tones used. Tones close to one another are calming whereas extremes in tone are dramatic.
Texture is also important. Texture in a painting should be used judiciously to enhance the focal point. Too much and it will look too busy which will detract from the subject matter.
Draw several thumbnail sketches exploring the tone, colour and composition of your subject matter keeping in mind the mood or narrative you wish to convey.
If you are unsure which colours would work try placing colour swatches or pieces of old paintings against the subject matter to decide which colours and tones will enhance the focal point.
Photocopy the painting and try out different backgrounds.
Scan or photograph your painting and try out different backgrounds in a programme or app.
If you have a piece of clear plastic or glass you can place it over the dry painting and paint directly on to it.
See the Texture Chart in my book The WATERCOLOUR Book : How to Paint Anything (available for £5 on Amazon) for some ideas for background effects.
How far you take the background is up to you.