It’s hard to draw any real meaning from the url color statistics. It appears that the dark gray works best, but it’s hard to say why. It does mask the advertiser url, but then the light grays do that even better yet do not perform as well. Standard blue and somewhat darker blue do very well.

That said, pairing the gray with the 336 x 280 format and center positioning is your best bet, as that combination of factors always outperforms the others in my data.

Use a dark gray url color and avoid unconventional colors.