The helmet flashes were regimental. Usually in the regiment's facing colours. In some conflicts the Highlanders would use a patch of the regimental tartan instead, but I can't remember if they did that in the Sudan in 1898 or not without looking it up.
30,000 of those odd grey uniforms were sent to British units as tropical wear in Egypt to replace their red uniforms, which during the Egyptian war were considered unsuitable for desert conditions (they were meant to be worn in the 1882 campaign, but did not arrive in time). It was a time of transition, and a permanent replacement for the red uniforms had not been decided upon yet. There were a few experimental false-starts.
However, some of the troops came to the Sudan from India, and they already had khaki uniforms so wore those (troops serving in India adopted khaki on a regular basis slightly earlier than elsewhere). So it's just a matter of whether the regiment had come from Britain via Egypt or if it had come from India. That grey uniform was not worn in Kitchener's 1898 campaign (by then the whole army fought in khaki), only in 1884/85.