Black on red, or the inverse, would work. So would black/purple or black/indigo.Eastern Europe has black be a metal, representing iron, hence why black and red are together in the region. But yeah, red and purple doesn't go well together.
That and I'm not sure the proper vexillogy for ERE flags well to replicate the design beyond the double eagle on red.
I wanted the crescent moon to be black to represent the Abbasids, compared to the white Umayyad star.
And I have to concur that Bataid use of indigo/purple might be difficult due to the price of the purple dye.
White/blue is a good color scheme too, that doesn't run into issues that red/purple does, and does not have the price problem of indigo/purple.indeed there are a lot of Marinid, Saadian and Wattasid banners that use white, green and especially blue as their main colours.