Zazzle uses a DTG (Direct-to-Garment) printing process to produce each shirt. Our light colour apparel uses single process printing, while our dark apparel uses double process printing.
Single Process Printing: The colour white is not printed on our light coloured fabrics. Any white used in the design will come across as transparent allowing the colour of the fabric to show through.
Double Process Printing: Dark colour apparel has an up-charge due to the extra process of printing a base layer of white ink under the design to make it more vibrant.
Even though the design is the same in all three shirts, note the colour differences in Cookie Monster's face. In the two light apparel t-shirts (white and pink), no white is actually printed in those designs (single process printing) and so you're seeing the fabric's colour filling in Cookie Monster's face. In the dark blue t-shirt, you're seeing the undercoat of white that was initially added to make the design more vibrant (double process printing).
Learn more about Designing for Light and Dark Apparel.