Below are the flags used on my command base for MacColla. They were made in photoshop and can be printed for use in your own armies. I painted over the printed design (see here) as I prefer the hand-painted look but that is not really necessary.
The yellow flag depicts 'the black galley' common in ancient West highland heraldry, especially amongst the MacDonalds and MacDougalls, the descendants of Somerled. It is typically taken as a representation of power and is also linked to the Lordship of the Isles.
The white sheet depicts the infamous "red hand of Ulster", a symbol linked to Ulster since at least the 16th century, and common amongst the heraldry of the O'Neill family. It is also found in west highland heraldry especially amongst the MacDonalds.
The flags are speculative, but the designs use symbols common in the contemporary 17th century heraldry of the MacDonalds and other west coast clans with links across the water to Ulster. There is no evidence that flags like these were carried by MacColla, or that they even existed, but both seem like fitting motifs for the Ulster-backed MacDonald.
I can't claim this as an original idea, it is actually borrowed from a description of MacColla in Nigel Tranter's "Montrose: Captain General". Although Tranter puts a white galley on a black sheet. I changed this to a black galley on a yellow sheet to be consistent with references to MacColla's colours at Auldearn, and to be faithful to West highland heraldry.
Below is a link to great website source on West Highland Heraldry. Some great ideas here for flags for your highland clansmen!
Apologies for repetition, but I wanted to include the flag information, previously published here, in this post too.