Since my preferred pen is deco color (since it's 
paint it doesn't fade) I've done outlines in 
white before.  Ok, so I do a lot more outlines in 
rapidograph ink because I've been doing more 
detailed stuff, but I still prefer the way a deco 
color pen handles.  I like to experiment with 
lines (some thicker, some lighter, some 
nonexistant) to give different effects.  I just 
did the oulines of a chobits cel where I did the 
body and dress in solid deco color lines and the 
hair in whispy rapidograph pens.  Since the 
colors will be mostly light throughout the cel, 
when it's done the hair will be differentiated by 
the lighter weight the lines give it rather than 
the color.  I haven't done a cel in all-white 
yet, because I'm wary of what effect a white 
outline will have on facial features against 
whites of eyes and pale skin.  

My best (complete) cel with white outlines was 
this deedlit cel.  Her hair is done in white 
outlines.