My 36 Year Old Son Experienced Loss Of Color In

My 36 year old son experienced loss of color in the upper end of his fingers. He had been outside hunting, but the weather was not cold and he had been in his vehicle warming them for several minutes. It took a while for the color to come back. He has no physical conditions. But this was disturbing and he would like to know what might be causing this. He said it had happened before but not to that extent.