If this is the 1959 scheme as applied to the C420s, the references I remember said that SP Lark dark gray and SP orange we're close to the 'original' colors.
Of course LIRR was notorious for crappy paint and accelerated weathering conditions, so it got to where 'no two cars were a common color' -- the thing to watch would be to get the weathered gray to match the weathered orange.
Remember that colors in 'daylight' on the prototype will need a different formula if to be viewed under artificial layout light...
For the LIRR orange and gray paint scheme, would just be reefer orange and reefer gray?