As mentioned before, i ordered from GMK a batch of single 1U R1 keycaps with the reverse color. Those will be used as branded merch on my shop, meetups and events but they were an excuse to test how the contrast with Pantone Medium Purple-C makes the reverse keys readable enough.
I have updated the main post with new photos of both keys, compared to DY as someone asked how the Pantone matches with the standard GMK color.