Why almost dye sublimation done on white substrates?

To understand dye sublimation and to obtain consistently satisfactory results from the process, it is important to understand the technology. In the simplest terms, dye sublimation is a method of transferring a printed image from paper onto a variety of materials, or substrates. There are many types of substrates that can be used, and they include textiles (T-shirts being the most common), ceramics (mugs), glass, plastics (key chains, identification tags, and decorative auto license plates), rubberized polyester (mouse pads, coasters), and coated metal, to name a few. 
Substrate is a term used to describe the base material onto which images are printed. Typical substrates include not only paper (coated and uncoated), but also fabrics, plastics, metal, films and foils. 
The reason why nearly all sublimation is done on white substrates is that dyes are actually transparent when sublimated. Therefore, white is the best background for the full color range to be visible and, most importantly, to exhibit their full vibrancy. Any other background may clash with one or more colors, often rendering them invisible, whereas white actually enhances all colors. For this reason, white substrates for sublimation are widely available and include polyperformance apparel, mesh-ball caps with a polyester front, polyester patches, ceramic plates, photo panels, mugs and tiles, mouse pads, jackets, beverage insulators, polyester calendars, clock faces, doll patches, polyester patches, plastic coasters, key chains and license plates, and coated MDF hard board (used in coasters and clipboards). It is possible to sublimate on substrates that are not white in color as long as the sublimation inks are predominantly darker than the substrate color. For example, black lettering on a red shirt. However, this is not recommended for photographs, logos, or detailed multi-color graphics as many of the image colors will be lost in the color of the substrate.
More info:
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