I am brand new to WW1 modelling, although I have numerous books on the war. I have recently ordered a 1/35 scale MK IV female tank made by Emhar, having assessed their kits by building a 1/72 scale whippet. In the case of the latter the colours were given in FS numbers. It seems the base colour should be a very pale gloss grey, but the box showed a sort of Khaki. I settled for Tamiya Olive Drab and dirtied it up with splashes of different browns to replicate mud. It ended up looking something like the picture on the box. Having read the forums I am quite baffled by the various colours indicated. However is there a commercially available colour from one of the manufacturers of model paints that would do as a base on my MKIV?
It seems to me that a lot of people agree on Humbrol 29 being close to the colour of the heavy tanks. They then add washes or other effects.
The male Mark IV in the museum in Brussels is still in a contemporary colour scheme, so gives a good idea of the colour (although there is no way of assessing if the paint colour has changed due to the passage of time) if you wanted to make your own mix.