Hi Ian,
Well there you go!!! "Murphy again" !!
If it's any help you could use the MM pattern on the vehicle and the vertical stripes on the limber and gun. Unit commanders were given a certain amount of licence as to how they camoed their kit, a point that turned up when I asked about using the vertical hard edge camo scheme on my M3 Scout Vehicle.
Other possibilities are also there, replacement limber and gun, desert yellow but not camoed up yet, repaired or recaptured kit etc, etc so I don't think you need to be stuck to one specific theme.
From every thing that I've read about the early campaings there was so much movement going on, attacks and success, counter attacks, retreats and counter, counter attacks any new kit was often sent straight into battle, probably camoed up in the field if they had time, or not as the case might be.
I think the early campaings give a lot of flexability in how you want to show the vehicles and kit.
It's the same with unit markings, regiments were moved about between divisions overnight and then moved back again 2 or 3 weeks later and so on.
Hope that helps a bit.
Cheers
Al
P.S. If your looking for a good decal sheet Fingerprint Designs have an excellent one for artillery units.