Mazal tov on your upcoming marriage!
Stains on white underwear or on a bedikah cloth that are clear, white, yellow, or light brown (the color of coffee with milk) with NO reddish hue are acceptable and may be disregarded. This is true even if the stain is larger than a gris.
If you are unsure of the size and/or color of a stain, it should be brought to a halachic authority for evaluation. You can ask your kallah teacher or the local Yoetzet Halacha or rabbi’s wife how this can be done in your location. If the authority rules that a color is acceptable, you should ask whether this color is always acceptable, or whether you should continue to bring stains of that color for evaluation.
Please feel free to get back to us with any further questions.
B’Hatzlacha!