How to remove the edge of shadeless object? I am trying to somewhat paint my object to a pure color. However, there is always a contour around it like 

On the top, there is mug that I changed to pure yellow. However, I can always see the black edge around it after rendering. Is there any way to remove it?

Appreciated!
I found the solution which is quite simple. 

Basically, when I change the color in Diffuse, I input the RGB in the range of 0 to 1 instead of 0 to 255. I guess Blender has some algorithm to handle it, which I am not quite sure.