The pre-made kits use sparkle novelties for the gradient top row. If you are starting from a keyboard without a FN row, or want legends on your top row, you’ll need to customise each key to have the right colours, following the steps in Edit an individual keycap (legend, colour) for each key.

If you want to keep the same legends, don’t change the template: instead, change the base color and legend color of the keycap as needed.

Example of gradient numrow with legends (Yuzu link):

Example of gradient QWERTY row with legends (Yuzu link):

Top row is for most 40%s, bottom row is for QAZ (I think gradient looks better with only 5 colours at that size).

In the Yuzu editor the yellow keys are quite hard to read, but I found in physical samples they are quite readable:

Lastly, here’s an old reference image from the KAT Iridescent days with examples of how the gradient can look on different types of keyboards to help visualise: