Depending on (a) how historically accurate you want to be, and (b) whether you want to use the same figures for other units/campaigns, the short answer is that the uniforms that are normally ascribed to the Continental Artillery were NOT worn by the gunners of the Northern Army in 1777 (and in fact were not worn by the four CA regiments themselves until mid-1778).
The Northern Army was woefully supplied with gunners in 1777 – of the companies that had taken part in the invasion of Canada, Bigelow's CT company had been disbanded at the end of 1776; Lamb's NY company was destroyed at Quebec in late 1775, whilst Hamilton's NY company was also disbanded at the end of 1776. Despite serving briefly as artillery commander of the Northern Army after his exchange in 1776, Lamb then moved on to command the 2nd Artillery Battalion/Regiment with the Main Army under Washington; as far as is known, all NY veterans went with him, leaving only a few of Bigelow's men in the Northern Department. So nobody in the fetching blue coats faced buff, I'm afraid.
The artillery battalion of the Northern Army was raised at the beginning of 1777 by Major Ebenezer Stevens, an experienced artilleryman (not sure of his State), to man the guns of Fort Ticonderoga and Crown Point. In June, it had 200 men, made up of drafts of infantrymen from the various units of the Northern Army; by Freeman's Farm there were 400, manning 22 guns; by mid-October there were 500, with 40 guns. The unit was not incorporated into the Continental Army until February 1778, and did not become part of the Corps of Continental Artillery until the end 1778, when it joined the 3rd Battalion/Regiment. This battalion/regiment did have the black coats faced red, but they were only issued that year (however, the OFM will be happy to hear that the unit had "a notable band"!). Stevens' men were the only part of the 3rd not drawn entirely from Massachusetts.
The original men of Stevens' battalion may have worn the uniforms of the infantry corps they had originally belonged to. However, given the state of clothing supplies to the Northern Army generally (even hunting shirts were rare), it is more likely that even they were in civvies. The first known official issue of clothing to these gunners was in mid-October, and this was only shirts, stockings/shoes, hats and small clothes.
Deserter descriptions for Stevens' unit, from February, March (three) and October 1777, list the following clothing:
- blue coat and leather breeches
- brown surtout coat and leather breeches
- as above, but wearing a fur hat
- short brown coat turned up with green
- blue coat faced white, red waistcoat, leather breeches, white stockings.
There is also a report of a deserter from Bigelow's company in April 1776, which might prove useful:
- short blue coat faced red.
Hope that helps.