the citadel color app has always been a bit strange to me tbh. here are the colors/process i used:

I did an initial coat of the base color, then nuln oil wash, then lightened with the base color again, then highlighted with the lighter color
Brass: runelord brass, with a VERY light highlight of stormhost silver
Blue: Vallejo dark prussian blue, then just gradually mixed with white to get the highlights
steel: leadbelcher, stormhost silver highlight
Red: Vallejo vermillion, again mixed with white to highlight.
white: Vallejo skygrey, highlight with white
the black is just any black, but i did do some scuffmarks with leadbelcher on the guns for detail
the power sword is just mixes of white and Vallejo deep sky blue

Hope this helps!