I prepared some hints for you before you paint your prints.
Here I show my three prints above. Two of them failed by wrong drawing and painting. Firstly, I used cheap (1-2$) water-based paint and it didn't even stick on the surface. It just leaked down throughout the material.
Secondly, I used a more professional one (13-14$) but I couldn't adjust the layer of the paint. What is more, the colour lost its shape while I was holding the keychain.
Finally, I used the same paint, but this time I changed some dimensions of the design. I gave a 2mm thickness to the Lambda and cut the shape 1mm down. It means, I made a path for my brush. Then I painted the lambda and the edge of the circle(key chain). I waited for it to be dry and lastly painted for one more layer. This time I lost the dark parts of the first paint, therefore the colour showed its real appearance.