I've done a fair few helmets now, and looked into this quite a bit - my understanding (and I stand to be corrected!) is:
1)
June 1935 smooth "Light Field Grey" (A.K.A. "Apple Green")
RAL 7009 and
double decals introduced for Heer helmets.
2)
27th January 1940 - OKH ordered helmets be painted in
smooth , matt, slate grey , still
double decal. This order applied both to factory production,
and was also intended to be applied to existing Apple Green helmets, which were to be repainted slate grey. It is unlikely that many helmets were factory produced (or field repainted) in slate grey, as just 7 or 8 weeks later a new order went out re helmet colours, as follows..
3)
21st March 1940 - Helmets were to be painted in
textured "Dark Field Grey" RAL 6006 with a
single decal. Existing helmets were to be repainted to this same colour and single decal configuration (although many weren't!). The texture used was Aluminium Oxide.
4)
28th August 1943 - Heer Adler decal removed - henceforth all helmets were
textured RAL 6006, without any decals.
NOTE ON PAINT SHADES:
As there wasn't the same technology back then, as there is now, to ensure one batch of paint was identical to the last, shades/tones varied massively between factories and even between batches from the same factory. You can see this well on many early "Apple Green"/RAL 7009 M35 helmets - the 3x liner retaining rivets were made at a different factory to the one the helmet shell was made at - they arrived at the helmet factory already painted in RAL 7009, so should have been the exact same colour as the shell they were added to, but quite often they were a very different shade - so there is quite a wide spectrum of colours that qualify for the loose title "Apple Green" - from a light grey/green, through a pea green, sometimes more grey than green, sometimes more dark green - not far off the later RAL6006.... Therefore, "Light Field Grey" is a more accurate description of the colour than "Apple Green". So, when replicating this paint now, there is no absolute "right" and "wrong"...!
Colour/decals by model:
M16/17/18/Austrian helmets reissued after 1935 -
1) Initially in double decal, smooth RAL 7009 "Apple Green"/"Light Field Grey" - when they were repainted at depots etc, they generally had their original WW1 liners replaced by the new liner used in the M35 helmet. You can see evidence of this on the outside as they have the large helmet retaining rivets, instead of the smaller WW1 ones.
2) After 1940, and right up to 1943, large numbers were refurbished with new liners (if they hadn't already received them) and given textured RAL 6006 paint and a single decal. These were probably intended for second line use, but are occasionally seen on the front line on original pictures.
M35 helmet
1) Initially double decal, smooth RAL 7009 "Apple Green"/"Light Field Grey".
2) From 27/1/40 should have been double decal, smooth slate grey, but probably rare.
3) From 21/3/40 should have been repainted with textured RAL 6006 "Dark Field Grey" with single decal.
4) From 28/8/43 Heer decal removed.
M40 and M42 Helmets
1) Initially textured RAL 6006 "Dark Field Grey" with single decal.
2) From 28/8/43 Heer decal removed.
Note on other services:
Kriegsmarine followed Heer regs re colour and decals on the same dates as above (although many field repainted various shades of grey for shipboard use).
Luftwaffe M35/40/42 4th Nov 1935 onwards smooth RAL 7016 blue-grey, double decal (M35s only), from 12th June 1940 (so, for M40/M42 helmets) rough textured RAL 7016 blue-grey single decal. Sometime after Heer orders removal of eagle decal in 1943, Luftwaffe also removes it's eagle from helmets, so henceforth Luftwaffe helmets have textured RAL 7016, no decals.
Luftwaffe Fallschirmjager M36/37/38 helmets Contrary to some popular belief,
Fallschirmjager helmets were never (as far as I can establish) painted in Lufty blue/grey - they were always painted as per Heer regs - so initially double decal smooth "apple green"/"light field grey", then briefly double decal smooth slate grey, then single decal textured dark field grey, then textured dark field grey without decals exactly as per Heer regs above, and from same dates.
Waffen-SS Seem to be as per Heer regs above, however they held on to double decals for a little bit longer, so initial, textured, "dark field grey" (RAL 6006) M40 helmets still had two decals for a short time before regs changed to single decal.
Also of note, the later textured "dark field grey" on SS helmets seems to often tend more towards a dark grey than a dark green as on Heer/Kriegsmarine/Fallschirmjager helmets
Modern paints to use for the above colours:
Personally I use the following;
For early war "light field grey" ("Apple Green") RAL 7009, I use Revell 67.
For late war "dark field grey" RAL 6006, until recently I "D.I.Y.'d" it, mixing 1 part black with 3 or 4 parts olive green (the other advantage of that is that no two of my late war dark field grey helmets are quite the same colour, which is quite authentic..!!) Some of my mates in GD Recon have also been off to B&Q and just asked for RAL 6006, and had it made up there-and-then in store...!! (You could no doubt do that with other RAL colours too..?) You can also get paint made up by people like
http://www.paints4u.com, who can prepare RAL codes -
just make sure to ask for it in a matt finish. I recently had my first tin of Matt RAL 6006 from them, and it is great.
For Luftwaffe "Anthracite Grey" RAL 7016 I use Revell 9 "Anthracite Grey".
Other useful colours for field repainted/camo helmets (these are generally the colours issued to armoured units for camo painting their vehicles - the probable source of most paint used on field repainted helmets):
RAL 8020 "D.A.K. Brown", I use Humbrol 93
RAL 7028 "Dark Yellow" I use Humbrol 94
RAL 7027 I use Humbrol 187 "Afrika Sand Grey"
RAL 6003 "Olive Green" I use Humbrol 117 or Revell 361 (or any old olive green)
RAL 8017 Red-Brown/Chocolate Brown - I have used Tamiya TS-1 spray paint, but this is no longer available unfortunately.
NOTE ON PAINT ON OTHER EQUIPMENT
It is worth noting that other items, for example the mess tin/cooking pot, were
not painted in the same colour as helmets - mess tins for example were initially in slate grey, and later moved on to olive green, possibly RAL 6003. It seems that it was the same with gas mask tins.
With regard to early "light field grey" / "apple green", as mentioned I use Revell 67 - here are a few of my attempts: