The "brown" R existed for less than a year in 1987. From 1968 to 1987, the Nassau/4th Ave special was always depicted as a yellow R in a diamond, with the bizarre seperation of the yellow and brown lines at Chambers St. which gave the impression the yellow R had it's own seperate platform on which it would terminate. The brown R existed for about 6 months, from the time of the N/R switch in Queens to it's elimination in November.
Also, no train was ever signed up in the brown R scheme. The R27/30 and R42's assigned to the line did not have them. They used the yellow designation right up to the end. I believe the only cars which have these signs are R68's.