According to Walton "Harbord's Independent Troop" was present in Ireland at late as June 1690 and the State Papers, Ireland (according to Sapherson) lists "Mr Harbord's Horse" (1 Troop) as part of the "Establishment of Horse, Foot & Dragoons for Ireland 1689/90".
As to why there was a hiatus between Schomberg's complaint in September 1689 and Harbord's arrest sometime in 1690 (possibly after the Boyne) the following factors
may have had an effect:
1. William had lost faith in Schomberg's capabilities due to his age and the slowness of the campaign and may have thought this was an excuse for inaction.
2. There was no police force or SFO to investigate such alleged crimes and governmental corruption was the norm for the period (even today it is a hard job to get sufficient evidence to get a successful prosecution).
3. The commissiariat had been completely overhauled by using a Dutch contractor to supply the Army. Isaac Pereira, a Saphardic Jew with a firm based in The Hague, undertook to supply 36,000 1.5lb loaves to the Army on a daily basis at a cost of £187 10 shillings per day, payable quarterly. This appears to have been working well by the time William arrived in Ireland to direct operations.
As to why Harbord's Troop was listed as being at the Boyne, it may well be that in was of the Army but not necessarily in the order of battle. The Army still needed local supplies and this may have been their role
I realise this could all be conjecture on my part
.
On the question of uniform - there were a number of units, some recruited in Wales(?), who had blue coats - Herbert's, Dering's, Bolton's and the Marines plus the Earl of Oxford's Horse. So you can take your pick - red, carmine, blue or even grey.
Sorry for being a bit long-winded and thanks for the welcome to the site.
Regards