(Quoted from PolyOne Wilflex. Full article here.)
While color can be created with ink that is pre-mixed or formulated with ink mixing systems, the substrate will always challenge the true weakness in all inks -- opacity. Ink manufacturers match color based on a standard printing parameter on a white cotton substrate. This is a necessity to maintain control of a standard matching requirement, but it falls short under certain circumstances. When an ink is printed on a dark substrate, or worse, a dark polyester blend, it requires a base plate.
Color is reflective light. Light travels into the color and reflects back to our eye the base and color of the sample we are viewing. We see good color when ink is printed directly on a white cotton fabric, because the ink absorbs into the cotton creating a base that is virtually the same color as the top coat. When the same color is printed on a white ink base, the color is not absorbed and the white reflects through as well. This results in a lightening of the overprint color.
To combat this, screen printers adjust screen meshes and printing techniques to deposit more overprint color onto the under-base white. Eventually, the color blocks enough light that it appears to be the correct color when viewed by eye.
The unfortunate twist to this solution is that different color ways need custom print parameters to achieve satisfactory results. For instance; gold, oranges and pastel colors need less film coverage than blue, purple and red hues. This is generally due to pigment strength and color formulas for each mix. Once the print starts in production, the ink shears, thus changing the opacity once again and requiring the technician to adjust parameters during the run.

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