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.