This came from the web site listed below. It actually has a picture for you to compare to if you'd like, about 1/4 of the way down the page.
"The incisors are colored with a yellowish pigment. The incisors start out white in the young rat, but by age 21 days the uppers have a slight yellow tinge. By 25 days the uppers are distinctly yellow and the lower incisors have acquired a little yellow. By 38 days these colors are more intense, with the uppers having more color than the lowers. The relationship between more pigmented upper incisors and less pigmented lower incisors remains true throughout the rat's life. In adult rats, the uppers are dark yellow-orange and the lowers are yellow (Addison and Appleton 1915)."
Orange teeth are healthy teeth. I have no idea why orange, but that's just the color of them. You'd only have to worry, oddly, if the teeth turned white. I think I read that's a sign of disease and/or malnutrition or a dying tooth, something of that like.