Here's a pic of the seat, this one from a low-back MkXIV at Duxford. the composite material is a reddish brown, rather like those tough, fibre board storage containers, and the back rest slip-on cushion is very dark brown leather.
If you have a look at Darryl's (Tony Hill) thread on his 1:1 Spitfire IX cockpit, you can see all the colours in detail, as very little changed in the way of internal painting and colours of fittings during the life of the Spit.