Why do dogs not like to see red pens?
Because you touch yourself...sinner.
Actually, dogs see in color. They see as a human who has red-green color blindness, so they see red as green and green as red and yellow as grey. other than that, due to a study done recently, they have both rods (associated with how you see at night) and cones (associated with what you see in the day) dogs and cats both see black and white but dogs see green, red, blue, and yellow…
is not true that dogs are completely colourblind. While dogs do not have the same color vision as humans, they are able to tell yellow from blue. Like a human with red-green colourblindness, they are unable to tell the difference between red and green http://www.certificate.net/wwio/pet0210010.shtml
truthfully dos are not colorblind they can see all colors. Dogs eye vision can get confused of colors that look similar, like red and orange they also see things a little darker or lighter then we do Dogs go much by smell, so not having color vision hasn't been required for their survival. In fact many animals other than dogs are (partially) color blind.