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).
So, the point is, it looks to me like the teeth can only really tell you if your rat is a full grown adult, which size should be able to tell you just as easily.