I will add some gradient colours to the background of this picture, so I create a layer, and instead of covering it in a uniform colour with the paint bucket, I use this light green gradient colour.
Then, in a separate layer, I use the paint bucket with a front white colour.
This means that in this moment I have three layers: On the first layer I have the picture in black lines; on the second layer I have the greed gradient colour, and on the third, I have a flat white background.
