Am I right in assuming that carotenoid pigments are the reason Cornus species have yellow and red bark? I (more or less) understand the photosynthetic function in leaves of biological pigments, but do they serve a similar purpose (if any at all) in the bark?
Other options to consider include thermoregulation, UV light resistance, and (if combined with toxins) browse prevention (as a visual warning to unpalatability).
I don't know what the actual pigments are in Cornus bark. The Cornus stolinifera stems are showing nicely red around here right now. There is no photosynthetic function carried out by these bark pigments, to my knowledge. Beta-carotene is part of the photosynthetic "machinery", but this is intracellular in conjunction with the initial capture of light energy mediated by the green pigment, chlorophyll. However, you'd get a better answer from someone whose plant physiology knowledge is more current that mine. Michael F. - what ARE the functions of these bark pigments? The explanations in your post don't sound like a good story to me? The most prominent bark colours that I am noticing around here are Salix spp. & Cornus stolinifera, I don't think that either of these are very toxic & I have seen both of them grazed or bark-stripped by animals in the past.
In Salix spp., prominent bark colours only occur in human-selected cultivars, so you could argue the function is to be pleasing to humans, with humans as the driving force of their evolution. As far as I know, the colours are natural in Cornus stolonifera; if they are not toxic, then seek some other reason. But maybe Homo sapiens-related evolution is active here too.
Over a year late to the conversation, but please see today's Botany Photo of the Day: Cornus sanguinea 'Midwinter Fire'