The site I have been using throughout the television section of the course is here.
The shadow mask makes sure that electrons fired from the filament responsible for each colour of sub-pixel do not hit the wrong phosphor dots, e.g. electrons sent by the “green” gun do not reach the phosphor dots that glow red or blue. This animation shows how the shadow mask works.
Remember: electrons do not have a colour. Colour is only produced when electrons collide with phosphor paint.