I just found this today. I have no idea how it evaded my attention for as long as it has… Repeated line alert!
Anyway, today’s example: Sea of Sorrows faction event. The filter wants Blue and/or Brown as shown:
BUT! We can’t ask it to show only Blue or only Brown. Because its locked in like that.
So I can’t do this (that’s taken from standard PVP):
What to do, what to do…
(I kid, but you likely won’t get an in-game fix for this for a very long time, if ever)
Wew that plug was shameless 
You caught me, I need to increase my revenue of $0 by tenfold or else my publisher will shut me down. 
If it helps at all, the easiest workaround I can think of for this is to filter by Base Rarity and start at the bottom, since that’s where all the mono-colour troops are. Same for weapons, except it’s only Legendaries that have multiple colours, from memory, aside from DB.