If you are pretreating a white or light colored shirt you must use IA for white or light garments. It is not just a watered down version of the dark. Also if you are still getting a yellowing on whites using IA white / light. Check your press temps and time. Only about 30 secs is required but if not dry press again in 10 sec intervals. Two things it must be cured with a heat press and dry. You can over cook the PT which will show as a yellowing of the shirt on white / light. That is the only side effect of to much cure time that I know of. Heat presses are notorious for not being at the correct temp. Mine is set at 340 and drops to 335 or so with a shirt being pressed. Also I suggest using the same amount of PT for white as dark. You can also still print white ink on the light colored shirts as an underbase if you really want the colors to pop and reduce fading and increase durability. I do, depending on colors, reduce my white ink lay down by 25 to 50 % when using it on a white or light shirt. Is it required no. Is pretreat required on a white shirt, no. Does it help improve colors, reduce bleeding or veining, increase durability, yes. Also on application be sure to brush all areas that have pretreat. If the area sprayed is outside the area of the press you should brush that as well, this helps prevent any crystallization caused by the steam escaping on the sides, this shows as a brown line at the edge just outside the area covered by the press, however it will wash out.