The Nick Jr Coloring book has all it needs for online coloring for your kid. It has tools like crayon, spray can, paint bucket, 3 brush sizes, eraser, several coloring pages accessible on the same screen, and a full color pallet. This should be your first choice for online painting.
This image has prominently sketched lines, easy for kids to fill the color in.
In this amazing online coloring page, you can click on the medium bowl, put in on the stone, and select one color material and put it on the stone, to make the paint. Now you can use your finger to make drawings on the wall. You can dip your finger back in the paint if it is finished. Use the medium again to create a new color for drawing.
You can paint this Igloo online and then print it on a paper if you have a printer. The white board is for writing your name on it.
Mushrooms from woods
The sea shell shelf
This page allows you mixing colors to make shades of your choice from the four basic colors. Splashing water clears the palette.
This page allows you to add colors and animated objects into the pictures. More fun than static color pages.