Color Blindness is an easily diagnosable condition. Many simple tests are available, some can even be taken online. Most are something similar to a cluster of dots. Inside the cluster is a picture or symbol made by different colored dots. To a colorblind person, all of the dots look the same and no picture is discernible. To a regular person, the picture is easy to see. An example is at this link:
http://colorvisiontesting.com/online%20test.htm#Test%20Card%20Number%203
A regular person should be able to see a brown sailboat in the dots. A colorblind person cannot see a difference.