I've never seen that before, but I found this:
"
A third pigment, or class of pigments, that occur in leaves are the anthocyanins. Anthocyanins absorb blue, blue-green, and green light. Therefore, the light reflected by leaves containing anthocyanins appears red. Unlike chlorophyll and carotene, anthocyanins are not attached to cell membranes, but are dissolved in the cell sap. The color produced by these pigments is sensitive to the pH of the cell sap. If the sap is quite acidic, the pigments impart a bright red color; if the sap is less acidic, its color is more purple. Anthocyanin pigments are responsible for the red skin of ripe apples and the purple of ripe grapes. Anthocyanins are formed by a reaction between sugars and certain proteins in cell sap. This reaction does not occur until the concentration of sugar in the sap is quite high. The reaction also requires light. This is why apples often appear red on one side and green on the other; the red side was in the sun and the green side was in shade."
Chemical of the Week -- The Chemistry of Autumn Colors
Sun does make them more red. I suspect that as they are TC they haven't quite acclimatised as of yet, once they do they will start to produce the red colouration.
PS is that bark he has them in? Also is that slow release fertiliser? Can't be much in the way of nutrients in that. To produce the proteins required for the red colouration the plant needs nitrates. No nitrates = no red. Maybe they are deficient.