If it ain't broke...
I personally liked the pseudo-noir (rouge?) effects of the previous entries in the series; it gave it a much more sinister look that contrasted hilariously with the comical nature of the plots. The newer colors aren't bad per se, just very typical flash and below the professional level. The old style wasn't perfect, but it was unique and worked GREAT.
The music is pretty much instant-classic and the voicework is extremely well done for the series as a whole.
Really, though, I think it'd be great if you kept with the old style. The opening shots and high contrast just look amazing and the animations are hilariously crisp. Even the extra grays added in #4 kind of take away from the feel-but it's cool to see you experimenting.