The Covenanter field army was well equipped with artillery. The army in England had an extensive artillery park which was employed successfully against the Royalist forces in the north. In Scotland, the mobile nature of the warfare precluded the serious use of heavier pieces, with only the ubiquitous frame-guns probably being deployed, outside of siege operations (i.e. Inverness).
Covenanter gun carriages were painted with pine-tar to help them stand up to the elements. Though not as colorful as some other guns of the period, this gives a fair contrast to the hidden grey of the infantry.
To get the effect I painted the carriages foundry ochre shade, then GW desert yellow (my middle shade had dried up), and highlighted with ochre highlight. The guns and metal fittings were gun metal on black. I liberally painted the whole thing with Army Painter medium shade which gave them a "creosote" look and the metal a rusty feel. However, the iron guns looked naff (and turned out to be ahistorical) so I worked up a bronze effect.
The two small guns are Covenanter Falconets, so the crew were fine. The larger gun was a generic Saker, so some green stuff bunnet's were fashioned. I couldn't really mix and match the crews as the ramrods etc were right sized to the guns. The crew were painted a broad mix of grey and drab with the odd flash of color on some tartan.
Bases are standard size 6 x 8cm gun bases from Litko, with Silflour tufts and static grass.
Final couple of shots show them deployed for action with the infantry.
All the best