Culumus clouds consist of water droplets suspended by rising air. They're dynamic structures in that water is vaporizing and condensing continuously.
Because culumus clouds are lower altitude and water droplets, they should be warmer than higher altitude icy cirrus clouds. To verify this, I took an image of a cumulus and neighboring cirrus cloud with my thermal imager. Below are the images for you to evaluate for yourself.
|The cirrus cloud is the crown on the cumulus cloud occupying most of this image.|
Notice the cirrus cloud appears blue and red, rather than yellow and white.