If you are buying the iron-on transfer, please make sure you choose the correct option. If you are going to be putting the art on a piece of clothing that is white or a light colour, choose Iron-on transfer (light materials). If you want to put it on a black shirt or something bright, then choose Iron-on transfer (dark materials).
The lighter iron-on transfer is slightly cheaper, but leaves blank spaces in the art for areas that are supposed to be white, so the art will take on the colour of the material behind it. The transfers meant for dark materials print on their own white background, so the art is always correct.