Edit: The second portal is finished. It's the green one in the photo above.
At the request of a friend, I created this Webway portal for his Dark Eldar army. To make it easier, I consulted the annals of the From The Warp Archives and found this article. Thanks, Ron!
The portal is made out of half of a knock-off Wiffle ball I bought at the dollar store earlier this afternoon. The portal isn't quite the right size, clocking in at almost exactly 2 1/2 inches, but it serves the purpose, nonetheless.
Most of the painting was done with spray paint I bought at my local Walmart. After roughing up the surface of the Wiffle ball with sandpaper, it got primed with Krylon grey primer. As always, I highly recommend it. After the primer dried (about an hour, just to be sure) I hit it with gloss black spray paint from Walmart. I don't know the brand name, but it's the stuff you get for 96 cents. Again, I let that dry for about an hour.
After the black coat was dry, I taped off a swirl pattern on the portal. After the tape was secure, I hit it with the gloss blue spray paint. More letting paint dry. After everything had dried, I broke out the large dry brush and went over the blue swirl with a stippling of Ultramarines Blue paint from Citadel to bring out the contrast between the black and blue colors. A sort-of highlight for the swirl was done in Enchanted Blue.
Next, I grabbed my old detail brush (recently replaced, and about time, too) and began making star-like patterns on the surface, more-or-less at random with Skull White paint. After this was done, I used the same color and brush to create a lightning effect on the surface.
Too easy, right?