Pinko is a critical term coined in America in 1925, originally to describe a person regarded as being sympathetic to communism, though not necessarily a Communist Party member. It has since been used as a derogatory term for someone who is (it is claimed) more socialistic than otherwise thought. Examples could be; someone on the political right who is seen as being 'overly' socialistic on particular matters, or someone on the political left who is (it is claimed) further to the left than he or she is normally known to be.
The term has its origins in the notion that pink is a lighter shade of red, a color associated with communism. Thus pink could describe a "lighter form of communism", purportedly promoted by supporters of socialism who were not themselves actual or "card carrying" communists. The term pinko has a pejorative sense, whereas 'pink' in this definition can be used in a purely descriptive sense, such as in the term pink tide.