hi Adrian...
alot depends on the actual colour you go for , you are correct in thinking that a RAL colour with the same code should be a good match with each other ......
but some colours are more sensitive than others ......
also ( i,m sure Fabby knows better than me ) but the way the person making the part can affect the depth of colour, i mean how much gelcoat is applied etc.
i should also say it can vary in the way the gel is supplied.... premixed or if the fabricator mixes it themselves with pigment,
what colours are you going for?
cheers
andy