I think Willow could be incredibly obsessive about this, perhaps even only using paper clips of a certain color, or according to a color-coding system...

Tara would use the color Willow liked the least because she wouldn't want them feeling left out.

Spike would say 'sod the paperclips' and go have a drink. :P

Xander wouldn't notice they were different colors.

Dawn would either a) use a paperclip of each color on every set of papers or b) rotate the colors she uses like clockwork: blue, yellow, green, red, pink, orange (repeat).