Darker colors showing through lighter ones can be 
easily remedied by just painting your layers 
thicker to begin with.

'patching' is what I call when you fix a cel 
error, like the splotchy parts in shura's cels.  
I've noticed the most on the flesh tones, but 
there's a big obvious one (to me anyway) on the 
front of Zel's hair.  If you're REALLY good you 
can 'surgically remove' an area of tainted paint 
(that's accidentally mixed/swirled/has another 
color showing through) with a couple of sharp 
tools and your nails and just re-paint it without 
showing up.  Last time I had a noticable patch on 
one of my cels was... oh... long time ago!  

but look, on the last sakura artbook cel I did, I 
screwed up royally on the arm.  I accidentally 
damaged the light color of the arm with... I 
think purple (accidentally stabbed it with the 
paintbrush while it was still wet)  You really 
can't see a thing, because I patched it.  I 
carefully removed a section of the arm and 
repainted it.  If you look REALLY carefully, 
you'll notice the shade near the top of the arm 
is a tad jagged and the line on the edge of the 
arm is a tiny bit faded from where I did the 
removal, but you'd never see it unless I pointed 
it out.  ^_^

It's easier than repainting a whole cel, wouldn't 
you say?  Good technique to learn!