According to Real Thread, tonal printing is a printing method that uses ink to achieve a hue lighter (tonal light) or darker (tonal dark) than the color of the garment.
Jeremy Picker, AMB3R Creative, says coming up with the perfect ink formula for tonal dark and tonal light is essential. Every printer comes up with this formula differently, but it's important to experiment with a wide range of garment colors to find the right mix that works on most. Using clear plastisol or a gel ink also gives a nice tonal or wet look.
The advantage of tone-on-tone ink is that you can use multiple garment colors in one run and avoid changing ink colors to achieve the perfect outcome. It gives the graphic a subtle tone and a soft hand, great for those trying to achieve a vintage look.
Consider using tonal light and dark in the same two-color design. It also works great with a one-color print. While multicolor prints are easier to design for, tone-on-tone prints give a professional look without the bold/opaque graphics that increase the cost of goods sold (COGS).
Explore tonal printing techniques to see if your audience receives it well. Positive feedback can help justify any small up-charges that are associated with specialty inks and methods.
Source: Jeremy Picker & Real Thread