What is the value of a colt 45 single action army sr. number 155xxx fair condition finish is brown grips are black checkered not wood the barrel is 7 and one half inches long last pat.date Jan 1875?

There is no SAA serial number that starts with SR (unless it was a custom order). It might be SA, but, it it is, then you don't have enough numbers in the serial number. What type of finish? What type of material are the grips made out of? What is the barrel length? Ballpark figure 500-700 based on your vague description.