How does someone with red-green colorblindness see yellow on a monitor/television?
Normal people see color due to the bellow mentioned combination of red, green and blue:



But due to genetic factors, The graph Distorts:



Now,If consider the normal Color code for yellow:

Yellow = R(255) + G(255) + B(0)

Or in other words

Max. Red + Max. Green + no Blue = Yellow

That means even if you can see a little of Red and Green, you can still a little deeper or lighter shade of yellow.

So Summing it up in the images:

 

Normal

 

Red ColorBlind

 

Green ColorBlind

 

Blue ColorBlind

Source: 

Coblis — Color Blindness Simulator

A quick introduction to colorblindness