It could be a good thing. When iguanas feel cold they darken their skin color when they feel light/heat, especially when you take them outside.
Can you do an experiment and take him outside when the sun is at it's highest? We're almost at the shortest daylight hours (the winter solstice), so that trick might not work. You will be looking to see if he turns darker or even black as a way to gather more heat.
Do you have a way to measure the heat of the vivarium? It should be 80F at the cool end and 95 or so at the warm end.
You can get a good non-contact thermometer to measure the heat of various objects in the viv, especially in the warm end. Iguanas are stuck with being the same temperature of it's habitat. A temp reading on his back should read over 80, but under 90F.
I have a Radio Shack non-contact thermometer that works fine, that I'm not using any more. I upgraded. If you can somehow give me an address to send it to, I'll give it to you for free. Don't worry, I have a good reputation on this forum.