Aha - this is the same behaviour as you get in an app like GIMP when you colourise
If you turn an image to greyscale, white is still white, black is still black, and a medium blue becomes a medium grey (in fact, we made the grey squircles by removing the pictogram from a Suru icon and then desaturating it). The same happens in reverse if you colourise a greyscale image with a blue colour in GIMP. The whites and blacks are unaltered.
When we thought the grey might be affecting the colour, I wondered if Image Majick was using a different process, but I think the colours were probably just a bit grey for some other reason in the early attempts? Looking at your new screenshot, I think this is a normal colourisation after all. So, we can stick with the greyscale squircles as a template, which is a lot more simple, and a pretty straightforward way of mapping the chosen colour to the highlights and gradient
EDIT: FWIW I think the latest screenshot is a big improvement on the current mixed look - there might be further improvements that can be made to the colours but the icons already look pretty great to me.