Hello, John & Merry Christmas!
I guess how you paint your guns really depends on what you are aiming to achieve with your armies. If like Ray, you are aiming to reproduce an historical orbat, then you are a little more constrained - thanks for that great info on the guns at Blenheim, Ray - Cheers!
If you intend doing battles other than "the big 4" WSS battles in Europe (at least from the Brit's point of view) then you can cast your net much wider - eg the smaller struggles between the Franco-Bavarians & Imperial troops of 1702-4 give greater scope.
If you plan using historical orbats as a guide for an imagi-nation project, though, then paint what you think you would like to "play with" - paraphrasing Jim - "your toys - paint them as you like them!"
Personally while I'm starting my armies based on the Blenheim OOB's and using Barry's very sensible guidelines for what constitutes a playable army for BLB, I am adding some units from other battles as, in the long run, I plan an Imagi-nation project based on the WSS but using a modified Games Workshop Warhammer map of "the Empire" vs "Bretonnia with some Imperial Allies" - the only fantasy element really being the map.
Much of my unit selection will be driven by interesting uniforms and also flags - so many tempting treats on offer from Maverick & Flags of War.
BLB with its small units & Barry's idea of the joy of Big flags makes this a feast, even if the majority of coats in an army are grey/white - officers, NCO's & musicians often added different colour!
As far as artillery goes, for example, I just had to have a 3pdr crewed by Danes in their violet coats with green cuffs even though there is no evidence of any such contingent being used by the Maritime Powers! So really, who can absolutely say that an Austrian short barrelled 3pdr or howitzer didn't make its way into Eugene's train of artillery at Schellenberg & Blenheim.
Afterall, what we do know about the OOB's, Uniforms, Flags for this era is probably less than what we don't know.
Enjoy & Cheers, Rohan.