Hematite streak test = red
Limonite streak test = brown
Jarosite = yellow
Nice pictures, Nick, I'd not seen the raws yet, thanks for the colors! Gonna stare at these awhile. So cool.
I think your reddish-brown powder may be from the iron rich spheroids. Like you say, only a small amount of would be need to produce this color. I don't think it's coming from the matrix.
Doesn't that matrix look soft to you? I should dig through the rock pile and find a piece of evaporite I know I collected in TX, (near El Capitan) and take it to a grinding wheel, see what it looks like.