How to reveal red patches in skin for a colour blind person?

Use the Black & White Adjustment layer. Move the red slider to the left all the way. Blemishes will turn black. You can use the spot healing brush on a separate layer between the original image and the adjustment layer to get rid of them. Skin that is just ruddy will darken smoothly and uniformly when you increase the red slider. 

The cool thing about this method is that it will help you find any areas with increased blood flow, even before it can be detected by just looking at it. You might catch a pimple as it's just beginning to develop. Maybe you'll find something that looks suspicious that turns out to be the early sign of skin cancer. 

@Beauteous_Aurora0D4D – To start, look at the individual R, G, B channels – green (magenta) will be darker in the pink areas and the blue (yellow) may be lighter in the same areas. The red (cyan) will be holding the opposing contrast which balances out the other two channels and these may be lighter or blown out as well.

Your info panel will also provide critical info, setting the readout on the right-hand side to either CMYK or Lab can also help to provide a different "angle" on the RGB values on the left. It is all about the numbers!

There are ways to exaggerate the skin tone variation, however, they may or may not help, it would all depend on your particular visual challenges.

hmmm does it work if you turn it to black and white photo?