How do I paint a drawing canvas slowly slowly ?

I am developing a colour identify game for kids, in which a brush comes on the screen and paint the whole screen with x colour in zig-zag motion. So, i want to show that the screen is painting only when the brush is touching that area.

Please Help

Maybe this demo will be helpful

howtoconstructdemos.com/roller-paint-demo

You can move the brush with Tween or a timeline.