A good strategy is to reinforce a shape or position indicator with a brightness difference.
For example traffic lights use red, orange and green but people who cannot see the difference can still learn their positions top/middle/bottom. Similarly, one can learn by shape - an X symbol means 'close' and + means 'add' etc.
Once the truth is learned it gets associated to a color, even if the color is wrong its still possible for the person to identify the correct traffic lights by color, as long as the brightness is not similar.
Most green used in signage and almost all walk signals and traffic lights is a very light tint of green (high brightness / white content) and the reds tends to be a medium brightness or light shade. That's not by accident, even a person who sees in grey-scale would still be able to distinguish between the two.
Understanding that opens up more options for you because you can still for example use the hues people have issues with as long as one is light and the other dark.