It's very difficult to separate artwork from music. I'd like to think I've done that when voting in this, (eg I would have had FOTD very high and NOTB top, although they're not my favourite albums). Although the artists didn't, as far as we know, hear the albums first, they will have been set particular guidelines to tie in with the whole package in terms of marketing. Something like: "Somewhere in Time, make it futuristic but with lots of references to Maiden's past." Or: "Fear of the Dark. Eddie doesn't have to be traditional, keep the design no-frills." The production of the artwork isn't completely divorced from the music and intended feel of each album. In one interview, Bruce talks about having his say on minor detail of the (unfinished?) TFF artwork, before realising he was getting into it way too far.
I find FOTD and NOTB the best in terms of colour, Killers and POM are a little too dominated by black/grey and yellow for my liking, although I do like the striking and plain Eddie-dominated design. Yellow does look hectic and maddening, like glaring artificial light, so it does fit POM, I just don't find it good to look at. I actually find Powerslave one of the worst in terms of colour - earth tone tan, light blue, pale yellow....not enough contrast all round for my liking, and colours that clash. Good contrast to what came before, though. DoD also has good use of colour, with the sharp black and white detail and dark/bright reds, but they screwed up with the design of those figures. AMOLAD is strange in terms of colour. The colours are a range of realistic colours, but there's an overall impression of brownness.