It is not safe to use colour photographs as a means to assess paint colour in a case like this as the following should make clear.
Firstly, to correct a few misleading statements that have been made here. The British had formally accepted Dark Olive Drab 41 as a substitute for Dark Green in February 1942, over a year before the paint colour standard for ANA 613 was issued. Even so Fig. 692b, Finish Specification Section 11 from Report No. NA-5724 (North American Aviation Structural Repair Manual) dated November 1943 clearly shows 'Dark Olive Drab' substituted for Dark Green on the Temperate Land scheme diagram for the Mustang. Anyone who doubts this can see this page of the manual for themselves here:-
By the time Drawing 109-00-268 was issued in the April 1944 NAA Erection and Maintenance Manual and Structural Repair Manual AN-O-60-3 the colour was shown as 'Olive Drab 613'. Ergo both colours substituted for Dark Green.
Both colours are Munsell "yellows" and the actual calculated difference between the OD41 and ANA 613 colour standard swatches as measured is only 1.66 where 2.0 or less equals a close match. Dark Olive Drab 41 has an ever so slightly more "greenish" appearance to some eyes, but with the variance of actual applied paints, weathering, etc., (and pigments specified for OD were listed as 'optional') this becomes largely academic and IMHO far too much is made of the supposed difference between them.
ANA Bulletin 157a of March 1944 listed 613 Olive Drab with Dark Olive Drab 41, Dark Slate Gray and Dark Green as the 'Nearest Equivalent or Superceded Colors'. This continued until Bulletin 157d of 1959 where there was no FS equivalent (from 595) for Olive Drab listed and the 'Color Standard to be Employed' was still shown as ANA 613.
In 1964 Bulletin 157e listed FS 'X 34087' for the first time as the color standard to be employed for Olive Drab, superseding ANA 613, with a note that chips for that colour could be obtained from the Coatings and Chemical Laboratory, Aberdeen Proving Ground. This colour standard was noticeably browner in appearance than both the previous Dark Olive Drab 41 and ANA 613 but has been attributed as a "match" for the latter which is where some of the confusion about this colour might have arisen in the past. The calculated difference between the 1964 FS 34087 and ANA 613 is 4.65 where 2.0 or less is a close match and the calculated difference between FS 34087 and OD 41 is 6.00. Visually the FS colour is significantly "browner" and if used as a "match" for ANA 613 would give rise to the impression that colour was "browner" than Dark Olive Drab 41. In his 'USAAF camouflage 1933-1969' (1969) Ross Whistler noted that "613 is substantially darker and greener than X 34087".
There are various technical pigmentation factors which also had a bearing on the development and visual appearance of these colours which I won't go into here.
Anyone who wants the actual measurements for these various colour standards - or indeed rendered chips of them - is welcome to pm me.