There are many hybrid variations between the red and yellow-shafted flickers. In the main photo you can see a typical hybrid –it looks mostly like a red shafted, but it has a red chevron on the nape (back of head) which is a characteristic of the yellow-shafted variety.
Yellow-shafted males have a black malar stripe (moustache) instead of red. Both the male and female Yellow-shafted Flickers have yellow under-parts and the red "V", or chevron shape, on the nape - these are the main differences between yellow and red-shafted flickers.
One trivial tidbit: The polk-a-dots on the belly range from 2-5 mm. The bird in this photo has the largest spots of any I have photographed, so we figure it must be sporting 5 mm dots.