According to the old Codex Arcanis, Beltine's color is yellow, probably linked to the cloying yellow incense mentioned in the PGtA's description of their initial handling of the bodies.
The old fan-run
onaraonline.org via archive.org had the following to say about Beltinian vestments:
Beltinian Vestments
- Alb: The Beltinian church favors a floor-length alb for its formal vestments, and priests that do not travel widely. Itinerant priests may opt for a 3/4 length (halfway between knee and ankle) alb for practicality, though ankle-length is still prefered. The color of the alb should be some shade of yellow, with lighter, airier shades being prefered in order to differentiate them from the heavier more golden tones often worn by Illiirites.
- Cincture: A sash of darker golden yellow or black, wide and often containing pockets. Higher orders may wear long sashes with tails falling at the hip.
- Footwear: Chosen for practicality. Slippers often worn by those that do not adventure widely, or for high ritual. Closed shoes prefered.
- Holy Symbol: A thurible or censer, burning incense when possible. Can be hung from the cincture when unlit. Sometimes also mounted to the head of a staff.
- Mantle Ankle-length, light yellow often trimmed in black. Hooded, and with extra fabric scarf-like at the shoulders, such that tails can be wrapped about the head, concealing the face. Worn open, but bound by the cincture.
- Underrobe: Worn when desired. Most often light yellow.
That was what was suggested for LARPing, at least. I have no idea how canonical (tee hee) it was.
The Beltinian NPCs mentioned in old modules (especially Enpebyn-based ones) are usually wearing yellow robes, though the "Gray Lady" epithet seems to have colored mod writers more recently, due to appearances such as the "gray maiden" who appears to Scipio and Darius in the Almeric description of the ARG.
Were I you, I'd go with the pale yellow, at least for any hood/mantle of the fig.