I think there's 2 ways to do it depending on how much time and effort you want to put in, a basic method and a more advanced method.
The basic method essentially aims to produce a smooth gradient from dark at the base of the tusk to light at the tip of the tusk. For this method I'd most likely base coat the whole tusk with Bleached Bone, then build up layers of dark gradient from the base of the tusk with several thinned down with water
washes of Devlan Mud, Badab Black, and Gryphonne Sepia, (just use the washes that give you the look you like the most).
You need to spread the wash thinly around the tusk from the base about 2/3 of the way to the tip. To get a gradient you need to get the wash to form a thicker "pool" on the surface near to the base with then drag it slightly along the tusk. With the washes being thinned it might take 2 or 3 coats to build up a dark enough coat. If you have a hot lamp you can hold the model under this to help the washes dry faster.
An add on for the basic method once you have you're gradient is to add some simple feathered lines from the base up the tusk. With thinned Scorched brown touch the brush tip at the base of the tusk a tiny little droplet might appear on the surface, you then drag this up the tusk as thinly and as straight as you can and it should draw a fine translucent line that fades out. Do several of these around the tusk and it should give a nice effect. If they look too red go over them with some thinned black wash to tone it down.
The more advanced method involves adding more feathered lines along the length of the tusk. Other colours that are good to use for tusk lines are, Chaos Black, Graveyard Earth, Khemri Brown, Dheneb Stone and Skull White.
I'm currently painting Gorbad (in my blog
), and haven't started his helmet yet, I'll try and take some step by step pictures of painting the tusks.