Firstly Signal Red IS BS381C-537. What is this 'RAL' that WZ877 refers to?
You are quite correct that the R/W/G scheme replaced the dayglo scheme officially from 1968. RAF Chipmunks were initially silver with yellow training bands during the '50s, then were silver with solid orange dayglo paint in the early '60s. From around 1963/64, the sticky dayglo paper strips replaced the dayglo paint as it was hard to maintain. In 1967, the silver was replaced by Light Aircraft Grey paint and the following year the R/W/G scheme was introduced although the first Chippy was not painted in this scheme until early 1971.