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The reason I gave up on those sort of things is that they make the keys much thicker (twice as thick) as well as increasing the size of the key tops both of which are a considerable disadvantage when you have a lot of keys.
These are some I have abandoned. Nail polish is far cheaper, adds no bulk and more effective in my opinion though it doesn't have the tactile differences of my older method (handy for finding the right key in the dark - however I note that those in your illustration don't have a tactile feature either.)