opaque transfer paper is for dark t-shirts and regular transfer paper is for white or light colored shirts.
opaque transfer paper has a white background that is not clear (opaque) so that the white in the design shows up on the t-shirt. This is because your printer cannot print white ink. Your printer has to use the paper color to make white. When you print on regular paper, you don't print white, you just print the other colors and the white in the design comes from the paper color.
Regular transfer paper for white or light garments is transparent so that the garment color (usually white) shows through and provides the white coloring for the design. If you use that transfer paper on a YELLOW t-shirt, all the white in the design will be YELLOW since there's no place to get the white from.
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or is there a type that the back ground white transfers on clear to dark t shirts? | |  | |  | |
Not any that you can print yourself on your home printer.
There are transfers that go on dark shirts that you don't have to trim, like plastisol transfers:
what is a plastisol (silk screen printed) heat transfer: a definition