Good to hear you're getting rid of the tw ....er, person !
Yep, the rectangle over the guns was a piece of doped fabric, a dull red to red orange colour, depending on the mix of the dope used to hold it in place. On the Hurricane, it was a single strip on each wing, as the guns were all together. On other types, like the Spit, it was a patch over each gun port, although the central pair on each wing sometimes had one patch over both. All R.A.F. fighters had these, throughout the war, even on cannon where the barrels protruded. They were to prevent dust and dirt entering the muzzles on the ground, but more importantly, to prevent 'wet cold' at altitude, which would freeze the gun barrels, and the working parts, due to damp, cold air being forced down the barrel.
The ground crews used to look out for 'their' aircraft returning and, if they saw the patches torn and smoke streaked, they immediately knew that, apart from the fact the guns had been fired, they'd need to re-arm and check over the guns as soon as the aircraft had taxied in and stopped.