My Grandfather had a form of red-green color deficiency that is carried with the x chromosome. Women ( xx) get this defective x chromosome but have a another "good" x to compensate for the "bad" one and are not effected. Men (xy) only get the defective x. Here's a link describing the whole process.
My Grandfather (xy) passed his color-blind x chromosome down to my mother (xx), who in turn gave one of them to me (xx) and one to my self-proclaimed "color-retarded" brother (xy). A few weeks ago we confirmed that I passed down one on my color-blind x chromosomes to Robby (xx). It's the gift that keeps on giving!
Robby can see red (in its purest form) with little problem, while my brother cannot. Robby cannot see green all the time. I have a feeling green and brown are two different names for the same color to him. Blue, orange, yellow, black and white he can see. Purple? Don't even go there! Orange causes occasional problems. When colors stray into shades and pastels they often get lost in translation to Robby.
Above is an Ishihara plate used to test for color-blindness. People without color deficiency see the number 8 while people with red-green deficiencies see the number 3 because they cannot differentiate between the green and brown circles.
Here, the majority of observers with red-green deficiencies see the number 73. The majority of observers with normal color vision see nothing at all! This is because normals' sense of color is actually masking the subtle brightness differences which color deficient observers use to see the number.
An example of how a person with a red-green color deficiency sees:
Robby can see reds, but the green and browns look a lot alike. Here's a great example from The Colorblind Photographer's website. Now I get why Robby's favorite color is yellow.