My planet has a star that peaks in the violet part of the spectrum. Its atmosphere has a purple-ish hue due to this. My planet has a 40.6° axial tilt, giving it frigid winters and hot summers. Now, the fleshy parts of plants, like leaves, are yellow, since the optimal pigment for absorbing violet light is Xanthophyll, which reflects yellow light. I want the trees on my planet to have blue bark with yellow leaves. What would be the optimal tree bark color on my planet, and is there anyway I could have blue trees?
If the cellulose (or generically any substance used by the tree for making the bark) happens to be arranged in a periodic way, it might behave like a photonic crystal and therefore selectively reflect certain frequencies of the spectrum.
This is why certain feathers or bugs seem to have metallic reflexes. The same can happen for bark, in principle.
So blue wood is really hard to do:
Copper Sulphate is a nice strong blue colour that dyes almost everything - it's also the active ingredient in those "root buster" products you put in your drains to kill tree roots getting into sewers. A literal tree poison
The blue / green wood you can buy at the hardware store is made by mixing Copper Chrome and Arsenic into the wood. These chemicals don't support growth - by design, it's a mould and termite protection.
So I see two ideas:
The tree is absorbing these chemicals from the ground and is able to do so as a natural form of protection against growth / insects / etc. I think this is a bit of a stretch - these chemicals kill trees as we know them, but these alien trees are not as we know them...
Coat the bark with a a creeper / vine like plant that's predominantly blue.
Eg some variant of Wisteria blue vine
Or Climbing Hydrangeas: