Yes, we can know what wavelengths of light a butterfly's cones react to. We don't know what they perceive. The reaction of sensory receptors is only the first step in perception. We perceive color. Is electromagnetic radiation colorful? Or is color an attribute of our perception?
As far as comprehending the conversation, a person who cannot perceive green, can be taught to properly use green in a conversation and under certain conditions understand another person's use of the term. My example was of how green contrasts with red. That is a perceptual issue, and if you can't perceive green, you can't give an honest opinon about it.
As to how color blind people can tell us what colors they can't perceive; it's a simple test. You can test their ability to distinguish various background colors against various foreground colors. For single colors, just use flash cards and ask them to label the colors; they will confuse some of the colors. An example of testing a foreground against a background:
Seriously? You're going to contest his assertion that a lizard can't understand a joke?
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