The on-demand production process prints your design's art colors as a combination of Cyan/Magenta/Yellow/Black inks.  Because your art is a digital representation of color,  there is translation between the solid color represented in your art and the printed color.  To give you an idea of what the printed output will look like, Pantone makes a Color Bridge swatch book which shows the approximate print translation from the uncoated color to the CMYK representation.  As an example, here is a green element from a .png art file:

The closest solid uncoated Pantone value for that green is 7481 U:
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And the related CMYK color bridge is 7481 UP:
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As you can see, the color shift is significant, which is due to the CMYK process inks not being able to represent the same pure pigment that is present in spot color inks.  Further, the translation will vary depending on the specific color/hue being specified.  That means that certain color values will be more or less directly translated to CMYK process.  Here is an example showing the 2248 U and 2248 UP (CMYK):
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The translation for this particular color value is much better.  So, the obvious questions would be "how does one prepare for the translation of spot color to CMYK printing during the design process?" We recommend using Pantone's Uncoated Color Bridge (https://www.pantone.com/products/graphics/color-bridge-uncoated) to compare the Pantone spot color versus the CMYK process printing match.  This color bridge is assuming printing on uncoated white paper, which is the closest approximation available for dirct-to-garment printing substrates.  As an alternative to the expensive color bridge guide, one can use pantone's https://www.pantone.com/color-finder#/search search feature to see what the U to UP comparison looks like:
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Additionally, the Pantone color libraries should be loaded into the Adobe design products, so one can use the PANTONE+ Color Bridge Uncoated library natively to specify a color that can be more closely approximated in CMYK process printing.  Of course, there will still be translation since we are printing on a dyed garment instead of pure uncoated white paper, but the translation should be much closer to the specified color.  

Lastly, in all this talk of CMYK printing, be sure that your art remains set to the RGB color mode.  Our printers will calibrate your RGB artwork correctly to match the printer profile.
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