1. Lift the colour
Using a damp tissues, cotton bud, or water loaded paint brush, gently apply clean water to the paint to be removed and blot off with a clean tissue or cloth. Some colours like Veridian green or Pthalo blue have high staining ratings and will never come clean.
Look up the watercolour's maker site for details about light fast and staining qualities.
2. Incorporate the mistake into the painting.
Some mistakes can't really be masked, like a fluorescent pink mark across a regency lady's white chemise. But many times you'll find that with some creative thinking, that black streak in the sky could be a bird, or that red spot in the grass becomes a ladybug. Put the painting aside if you can't think of anything at the time, you may be surprised that the 'mistake' isn't as big as you think it is when you come back.
3. Fix the problem with another art medium
Change media and turn it into an acrylic painting. Try ink, charcoals, coloured pencils
Kinuko Craft is an amazing artist who paints oils over watercolours - see here techniques here The Art of Kinuko Y. Craft
4. Prime over the top of the piece of paper and use as a ground for another painting/ drawing
No reason to throw away good paper! Use if for experimentation! The Sentinel is acrylics on watercolour paper.
5. Cut it up and use it for a collage or assemblage piece.
6. Flip the paper over and paint on the back (this will depend on how buckled the paper is and how heavy you are with the washes.)
7. Scan the painting and fix it digitally.
Ok you're not going to have the original, but it may be the beginning of a beautiful new painting. My piece Contemplation was rescued after I made a number of errors in the watercolours. I've now started working digitally in a way that allows me to use scraps of sketches as the basis for paintings like Teal
8. Cut out unpainted sections and use them for small artworks such as ACEO cards or OSWA paintings. ACEO's are 2.5" x 3.5", while OSWA normally are around 4 x 6".
