AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAGGGGGGGGGGGGGGGHHHHHHHHHHHH!!!!!!!!!!

Depends on the underlying paint and clear coats that you're using.

I use Tamiya Acrylics for all my painting, seal it all up with a nice coat of Future Floor polish (it's just clear acrylic). 

To get some dark color into the panel lines I use artist oils and turpenoid.  I take a clean bottle cap (like a twist off cap from a water bottle), put about a BB sized blob of artist oil (usually burnt umber, sometimes Indigo depending on how dark I want it to be) and then fill the rest of the bottle cap with Turpenoid. 

I then liberaly "wash" the mix all over the plane using a wide, flat brush (anywhere from 1/4 inch to 1/2 inch wide) and slop it all over.  Most of the color will be deposited in the panel lines and around detail parts.  This will also leave some color on the panels, so if you want to you can wipe it off the panels using a tightly folded and compressed piece of paper towel.  You want the paper towel to be compressed so the fibers won't also pull the color out of the panel lines.

Or, assuming the cost of oil paint and turpenoid is out of reach, there is also the colored pencil route.  They come in all colors, including metallics.  The lead is a little softer than your standard #2, but with a nice sharp point, it really gets into those panel lines. 

Here's the silver one I got to do paint chipping

http://www.samflaxny.com/browse.cfm/4,7072.htm   (or if the link doesn't take you there, just search for metallic silver)

But for the panel washes, you'll want browns, dark greys or blacks for lighter color paint jobs and lighter greys or browns for darker paintjobs.

-Fred

You can get a tube of burnt sienna, for example, for 6 to 10 bucks depending on where you buy it, the quality of the brand, and the size of the tube.  Even a small tube will last you a very long time.  I'm still working off the original tubes I bought over 2 years ago, and I've barely even put a dent in them.  They'll last me at least another 10 years.

Turpenoid runs for about $4 to $10 depending on the size you buy.  I'm still using the original bottle I bought over 2 years ago, and have about half the bottle remaining. 

Talk about bang for the buck!