I practiced with a couple different things and 
found one that really sings! ^_^  First, keep in 
mind that I work at a vet and second, that I used 
to be an art major and so have lots of random art 
supplies laying around.

I mixed the right color paint, then thinned it 
down to almost the consistancy of ink. Then I 
found some old dried up ink cartridges to my 
calligraphy pen (which also has a tip for single 
width lines), I cleaned them and used a needle 
and syringe to inject the new "ink" into the 
empty cartridge. Voila - I could now paint thin 
lines of the exact color I needed.

One warning - the thinned paint seems to take 
forever to dry, and it can dry "flaky" looking if 
you thin it down too far. Also it almost gummed 
up my pen which was not fun to clean afterward, 
so I recommend NOT using a good pen. o_0

Many Sharp Smiles,
--Drac