I took advantage of the keyboard I made with keycaps from 7bit grab box thread to try water-slide decal paper.
Not as good as doubleshot but can help for custom layout.

I used transparent decal paper for inkjet printer.
After printing the layout and a varnish spray coating

each keycap mark is cutted apart

After a few seconds in water, the thin plastic sheet is applied on the keycap. A warnish coating is then added with a brush.

On this first attempt :
* some bubbles are visible, mainly on darker keycaps. Use of "micro set" and "micro sol" could have been interesting.
* edges of some labels come loose a little;
* with a particular angle of view, edges are visibles;
* a few mistakes (F3) and alignment not perfect;
* this method is only valid to make labels darker than the keycaps.
Next time will try dry tansfer decals.