Best way I have found is as follows:
Make stencils/or tape up
Then to keep this simple, say you want to apply red on base white
Once the stencils/tape are in place - in the area you want the colour to go, seal the 'gap' where the paint bleed is by applying the base colour white along the edge of the stencil/tape.
This will then seal the area for paint bleed.
Have fun
