Up to 9% of the male population has some form of dyschromatopsia. They don't see red, green or blue in the same way everyone else does. Sometimes more, sometimes less intense. This gives rise to the following advice: Don't rely on colour alone to communicate. Do you think green is a good background colour for a success message? Everyone understands green after all. Wrong. Same goes for red. You should combine colour and - for example - iconography. So in our example that could mean you'll combine the green background with a happy smiley icon or a thumbs-up icon, just as long as it’s something that indicates a success or positive feedback.

example of communication with colours
Don't rely on colour to communicate e.g. success or failure of a transaction. Combine the colour with words or even better with icons.

Do you want to see what people with dyschromatopsia see? There are plugins for your browser, like e.g. I want to see like the colour blind for Chrome. In that case you can check your page, but it means your page is already up and running.

example of normal colour vision
This is the same example as seen above. This time in Chrome with the plugin "I want to see like the colour blind" activated. The plugin will appear in the upper right corner of the browser.
this is how a person with protanomaly will see our example
This is how a person with protanomaly will see our example. These people have difficulties in seeing the red colour spectrum. Since it's an anomaly this means they are not blind to red, they just don't see it "right".
this is how a person with deuteranomaly will see our example
This is how a person with deuteranomaly will see our example. In this case the person has diffuculties to see the colur green in a correct way. Approximately 5% of the male population have this gen defect.

What about testing during the design process?

Good news for Photoshop users: Just go to View > Proof Setup > Color Blindness where you can activate one of the two filters to see how your design looks to people with different types of colour blindness.

settings for testing color-blindness in Photoshop
In Photoshop you can easily test your images for Protanopia-type as well as for Deuteranopia-type.

If you're using Sketch, you can add a plugin like Stark and now you can check for different types of colour blindness as well. You can even check for different kinds of colour blindness. Go to Plugins > Stark > Simulate Colorblindness > ... Stark can also check the contrast of two layers (background and foreground). That’s very handy!

settings for testing color-blindness in Sketch with the Stark plugin
For Sketch you'll need a plugin called Stark. It can test your designs for colour blindness. It can also perform a contrast check for you.