So that means that to make the zero in colour, (by making two copies of the monochrome glyph and deleting a contour from each copy before colourizing each glyph) that I need to change the direction of the remaining contour of one glyph.
This is just a simple example to discuss the issue: the issue arose when I was making a seven-colour design starting from a monochrome line drawing.
I have now tried a test font and used Font Validate… to put it all correct.
We recommend all simple glyphs have correct contour directions, so if you remove the outer contour, you should adjust the direction of the inner contour.