It is an anathema to say that water colours paintings can not be corrected. They can but the result may not be satisfactory. Place the whole painting under running water, scrape away with a Swan-Morton Knife (TAKE CARE WITH THE PAPER!), and/or gently lift the colour with a brush, tissue or sponge. Hard edges can be removed by wetting and sponging. Opaque white paint (Chinese White) may be applied.