Choose a color from the palette and click the canvas to start painting. Click inside the canvas again to turn off the brush.
You can also experiment with different color modes:
Use Random Colors for a psychedelic multicolored effect,
incrementally Darken the selected color or
choose Ligthen to get the opposite effect.
If you want to change the size of the pixels, a greater number of columns will generate a canvas with smaller pixels and vice versa.
Have fun!