While you CAN use Light/White transfer paper for pastel coloured garments, your image can end up being tinted with the colour from your shirt. For example, if you have an image of a banana and you transfer it onto a light blue tshirt, the yellow in your image and the light blue in your shirt may mix together to leave you with a green-ish looking banana.

If you want the purest colour out of your image, it's best to stick with a white garment or only black colour text.