Drawing above Canvas in Game using render target

Hay, After Watching many tutorials about drawing material on render target method in UE4 i have been able to draw in game, but the thing is that when i try to change the color it changes for the whole and previous drawings as well, what i want is something like this:

This is the Canvas material i use:
How can i change the color without effecting the color of the previous drawing i made.Please help. I use just Blueprints.