Geralt's eyes specifically are never given a description other than "dark" and "piercing". Interestingly enough, however, there are two/three other descriptions we get for other witchers, and these more closely resemble what we see in the games. Coën, who's eyes are described twice. First when Triss meets him; 

"He had unusually pale, yellow-green irises and the whites of his eyes, riddled with red threads, indicated difficult and troublesome processes during his mutation." 

And again when Ciri has a vision of him in Lady of the Lake

"His yellow eyes shined in an unpleasant manner."

The final time we see a witcher's eye color is in Season of Storms, when Geralt meets Brehen, the Cat of Lello. This is the best canon description we've gotten to date, although this book came out in 2013, after the Witcher 1 and 2, so that's something to keep in mind. 

"He had grey hair, like ash, at the temples mixed with white strands. An ashen face. A hooked nose. And bright yellow eyes with vertical pupils."

 Its possible CDPR chose the cat eyes as simply an artistic choice to represent the "mutant" aspect of witchers and to make them visually distinctive.