Look at the model layer by layer, the gradient is created by having a thin layer of color on top of white/black.

Yes I checked the project , that's why I was surprised to see a full color layer at layer 1 & 10 layers 2 & 9 , makes a small white & black squares layers 3 to 8, makes a big white & black squares Ok was not thinking properly, it's testing the transparency of a filament or how to create variations darker/lighter of a given color this test is reversible ( same on both sides ) and shows the effect of 0.2 and 0.4 mm of the color on top of white or black ;) In fact we could do the same test with other color combinations, which ( with thinner layer height ) could then be a good database for HueForge prints :) Sorry for my initial miss understanding

no worries, you've got it! I made these to help design keychains that use dark/light versions of colors while I still only have one AMS. As for other color combos, my Chikipi uses a thin layer of yellow over red to create orange https://makerworld.com/en/models/157971#profileId-173378