A day in the life of the kuesters uses two primary colors and lets the kids mix them together to learn about color creations.
Good for ages 1 and up.
Get the instructions here:
http://adayinthelifeofthekuesters.blogspot.com/2012/10/toddler-activity-sensory-paint-bags.html
Learning at his feet uses several bags of just one color for sensory play.
Get the instructions here:
http://learningathisfeet.blogspot.com/2012/08/the-toddler-diaries-paint-bags.html
Cuties by zootys combines all of the colors into one bag and lets the kids discover the classic yucky brown.
Get the instructions here:
http://cutiesbyzootys.com.au/monday-make-it-squishy-paint-bag/
Sweet happy life put the bags in the window so the kids can see the sun shine through.
Get the instructions here:
http://www.sweethappylife.com/2012/activities/toddler-games-activities/toddler-indoor-activities/sunny-window-paint-play/