.....Shades of Grey.......
Dulite is a proprietary Oxide finish.
Read all about the company (and products) concerned here: Du-Lite Corporation
FN (or the Belgian Army) used that grey "paint" finish on their Model 49 self-loading rifles and the Model 50 .30-06 "Mauser 98" rifles: I have owned both.
Whether that finish was applied when "new" is up for discussion, as all of the 49s and 50s I have seen appear to have had their Belgian crests ground off the receiver ring.
I also once owned a Model 49 that had been painted in what looked suspiciously like the classic black "Suncorite".
To get "black" Parkerizing, add Manganese to the brew, for a grey finish, add Zinc.
Painting over Parkerizing is common; see L1A1s in general.
If the "paint" has genuine examiner's etc stamps OVER the paint, it may be have been applied at the factory. If the paint covers such stamps, it is probably done at a later date.
your mileage may vary.