I don't understand why rattle can paint would fade less than paint in a 4 litre can but...... it could depend on the thinners used and the percentage.
The Protec spray cans have the military IRR treatment in them. The 4 litre tins don't, presumably because the regime dislikes civilians spraying their vehicles with paints that don't glow under active infrared lighting like those in use by ANPRs, mobile phone/seatbelt detection cameras, toll points and such.
Source 1: I have tested objects painted with both the spray paints and the 4 litre tins under active IR illumination with a Gen 3 night vision image intensifier. Spray can paint doesn't glow under active IR light. The other stuff does.
Source 2: I have wolf rims sprayed camouflage brown by a well known retailer which have faded to pink. The spray can-respray (all over camouflage brown) which I did the same weekend as I fitted the wolf rims is still camouflage brown and only slightly faded.
I'm glad you were able to discover through experience that the camouflage brown Protec spray paints are indeed the correct colour for your FFR. It often pays to test these things before opining that a fellow Perentie/mil LR enthusiast is outright wrong because the graphic on your spray can looks like it is a different shade particularly when the label states "lustreless" but the label is glossy, which may have been a clue that it may not be an OCD-level true representation of the colour of the paint in the can.