The Gray-Green uniform was standard, the khaki was for overseas "tropical" service, most notably in North Africa.
Units that went over in the pre-war period were pretty completely equipped for their tropical assignments.
Units that went over after the disastrous losses in 1941 and particularly 1942 were often not completely equipped for their assignments.
Some units went over piecemeal, one battalion or battery at a time. In these cases again the re-equipping with tropical kit was hap-hazard. You could easily make the case with your artillery formation -- shipped over in response to dire need, before their parent regiment / division could be re-equipped with tropical wear.
And replacement materials were also not entirely tropical kit. So even if a unit started fully equipped with tropical wear (ie: the sun helmets), those portions that wore out more quickly (ie: the pants and shirts) might well have been replaced by non-tropical replacement kit.
So … if you put sun helmets and Continental uniforms on your soldiers it is not by any means wrong, but it does indicate they are probably late arrivals to the theater. Whether that means Tunisia (likely), or El Alamein (less likely, but possible), is up to you.*
-Mark
(aka: Mk 1)
*Note: Well, of course it's up to you in any case. But if you want a credible explanation for their appearance, you have one for 1942/43 use.