Each RAF squadron (and airfields, sector stations etc) had a call sign of a two syllable word with distinct vowel sounds to be easily distinguishable over the radio - for example 92 Squadron was "GANNIC" during the Battle of Britain, Kenley was "SAPPER". These were used when the sector stations communicated with squadrons to direct them, e.g.:

"Gannic leader, this is Sapper. One hundred and fifty plus approaching Dungeness at angels twelve. Vector 120. Over."

"Sapper, this is Gannic, message received and understood."

"Gannic, bandits include many snappers. I say again, many snappers, keep a good lookout. Over."

"Sapper, this is Gannic. OK, understood. I am steering 120 and climbing hard through angels seven. Over."

(From Geoff Weullum's First Light)

A squadron of 12 aircraft was subdivided into sections of (at the time of Dunkirk/The Battle of Britain) three aircraft (the RAF later moved to four aircraft sections, as used by the Luftwaffe from the start of the war). Sections were a colour (usually Red, Yellow, Blue and Green), with the aircraft numbered 1, 2 and 3. There's a nice illustration in [an article on the Classic Warbirds site] (http://www.classicwarbirds.co.uk/articles/royal-air-force-tactics-during-the-battle-of-britain.php). When communicating within the squadron it would typically be as "Red 1/leader", "Blue 3" etc.:

"Watch that bastard behind you, Yellow 2. He's close, boy, real close. Break Yellow 2, break you clot, break!"

"Blue leader, can you give me a hand? I'm just off to your starboard somewhere."

"Sorry, old boy, I've got three 109s on to me but don't worry, I've got 'em surrounded."

(First Light again)

The two could be combined to identify aircraft within a squadron ("Sapper, this is Gannic Red 2, over"). In the case of Dunkirk, with only three flying Spitfires available, there's only one section so no need for colours, though really Fighter Command were mostly operating as pairs of squadrons over Dunkirk.