Today, I'm working on a technique in which I'm applying a grayscale image to the surface of an object by adjusting the travel speed for that surface. I'm printing in a Shaxon natural PLA which usually results in a part which is honestly as clear as ColorFabb's XT-Clear from my experience.
For the image, I'm applying a harlequin mask in such a way that it's toggling speed between 15mm/s and 55mm/s on a teardrop-shaped vase. Given the way semi-transparent materials flow, this usually results in a difference in opacity, creating controlled surface imaging.
All this is based upon Mark Wheadon's original work in the area.
I've done this before, it was quite fun, I did a vase mode lamp shade and applied a flame around the perimeter. I used a translucent blue and it came out pretty good. I also did a rocket and applied a name to the side. That one came out a little weak, which seemed to be the biggest issue I ran into. Part strength after it's done is a little iffy due to layer adhesion differences.
I've run this vase something like four times now with different upper-end speeds. The best one is 20mm at the most in height, all others ended up in "air prints". It's subtle but I can definitely see the design in the surface. I'm thinking that one of the translucent colors as you've indicated is a better way to go on this. I found that I had to crank up the temperature by 10 degrees for it to do this well.