Where is yellow?

We are having painting session this evening, actually not really painting as I prefer to mix the brush with colors and mix it into the small water pail. 1st I choose white and after adding into the water it changed the water white. Then I choose blue and the water changed blue but when I choose red it changed to purple, I am a bit puzzle but mommy told me red and blue turn purple. Next I choose yellow and it dissolve into the pail and I can't find it. I ask mommy where is Yellow? Mommy say it gone... 没什么说服力 hor...