The inside of my mk1 carrier spare barrel stowage box also had that semi gloss brown/green color on it. I thought that it might have been the il from the canvas or the polishing effect of the canvas on the Derk's seat frame, but the semi gloss seems to work right around the tubes of the seat frame.
A semi gloss paint is certainly a lot less porous than a flat color. Personally, when I am using automotive paint on my carrier, I simply use a lessor level of reducer (such as paint thinner) to flatten the colour from the gloss paint. It's a lot cheaper than reducer, and a heck of a lot cheaper than the flattening agents.