If we can trust Henriques' comment (and he gives a detailed account of many aspects of the early tanks), it would appear that the Mk.I's at Elveden had a lot of green colouration, perhaps multiple hues as are seen in the Hassall painting. The "pink" colour was likely a reddish ochre shade, which can sometimes be seen in a vivid sunset. However, neither he, nor Robinson, mention any green in the colour scheme repainted soon after arrival in France. A Bavarian non-com, observing the tanks on Sept. 15, commented that they were "grey with green stripes, forming the impression of a house of reeds". The limitations of his point of view on the battlefield must, however, be considered. A drawing which accompanied his report depicts a scheme which does not resemble any photo of Mk.I. camouflage. The best evidence I have seen suggests that the tanks were repainted in a scheme of yellow ochre, grey, and multiple shades of brown with black bands or stripes. However, the tanks were repainted in haste, and may have retained traces of their Elveden colours. The wood-framed grenade roofs were fitted in France, after the tanks had been repainted. The wire-framed roof on the Elveden tank was a unique type, not seen in France.
A number of the models show the hull roof, sponson roofs, inner rear horns, and steering tail as camouflage painted. I have not seen a photograph which would support this. (The hull and sponson roofs, inside of the horns and the whole steering tail thus do not appear to have been painted with the camo pattern - presumably, the basis over which the camo colours went was the medium, or lightish, grey oft-times mentioned, and thus the colour of the hull and sponson roof etc.) The tanks were provided with a painted cover for overhead concealment when bivouaced. Cecil Lewis (in Sagittarius Rising), flying with the RFC over the Somme in 1916, comments on brown tanks carrying red petrol tins, but it is not clear that he is specifically describing their roofs.
While the photographic record is far from complete, and a moderate degree of variation exists in the "Solomon" patterns, I don't think that "anything goes" when depicting this camouflage. A number of identified Mk.I's have photos of at least part of their scheme, and the unphotographed portions can reasonably be inferred from photos of other tanks, particularly those within their own company.
According to Stephen Bull, the Royal Engineers kept stocks of burnt umber and yellow ochre paint. "Burnt umber" is close to Methuen 6F6 (Vandyke brown) and FS595a 30099 or 30118. According to Urban Fredriksson's colour reference charts, which are posted on the IPMS Stockholm website, the Humbrol match for 30099 is: 8x Hu110 + 2x Hu113 + 1x Hu33. The match for 30118 is Hu142. "Yellow ochre" is Methuen 5C7 and FS595a 33434. The match for 33434 is: 4x Hu154 + 3x Hu94 + 1x Hu 34. Looking at the Benson and Topham paintings, the grey seems to be a neutral medium tone.