I would suggest muting the colours rather than going pastel for a loli-inspired Joker. Go for a dark purple and a more forest green than the vibrant purple and green! he has. Orange... is really hard to find in a colour that isn't bright and vibrant... try for a rich and reddish orange, it will make for a lovely highlight if everything else is muted.
Use a dark green wig and put it into pigtails or little buns.
The Joker often has a top hat, so it would be adorable if you had an itty bitty one pinned into your wig... maybe use a little white bow that says "bang" on it.
A lot of elite-lolis will get up in arms about this, but fret not. You are being very creative and awesome.
I *adore* the idea of lolifying characters, especially non-anime male ones