This picture made in 1954 when Velazquez had an obsession with the portrait Pope Innocent X:

He said he never saw the painting although the pictures are very similar. They say this supports his belief "we are all carcasses."

The picture has a lot of dark colors. The brightest colors are in the "beef". The "pope" has a grimacing/screaming expression.
I think the "beef" also looks like a face. I do believe that the picture has a lot of intentional similarities and differences to Velazque's portrait. I think the use of dark colors is for drama, its moody, dark and almost satanic. I this the expression is ironic because the pope is supposed to be well composed and in Bacon's version he is not. I really like Bacon's version because it was daring and sassy.