Thanks for the reply. I experimented with different themes and color options and eventually discovered that the Colorize option works fine as long as I don't enable
"Colorize by shifting hues instead of tinting"
Both features (tint and hue) are working fine in this release, Victechnical, but they do very different things. I'll repeat what is written above:
"The Tint colorization method first converts an image to grayscale then applies a tint to the image. Blacks remain black but Whites become tinted with the new color. Original color information is lost.
The alternate Hue colorization method first determines the dominant color of the image then shifts the hue of the whole image so that the new color becomes the dominant color. Unlike the tint method, Whites are also preserved along with Blacks, but all other colors are shifted as well.
Which method will work best for you depends on the theme and your own personal preferences, but at least now you have a choice!"
Hue shift leaves blacks, whites and grays alone. The effect is a lot more subtle than Tint, so if your dock background is mostly gray it might look as if it wasn't being colorized at all (which is, perhaps, why you thought it wasn't working). But use a dock background with actual color and you will definitely see the difference (have a look at the pictures in the first post, first picture is the original image, second the same image tinted to red, and third the image tinted to red via Hue shift).
The other important factor is what Hue Shift does: it shifts the Hue of *all* colors in the dock background so that the *dominant* color has the same hue as the colorization color. If you have a dock background where the dominant color is green and you shift it to red (i.e.; rotate all colors in the bitmap by -90º) then the greens will become red *but* any blues will also become green. This is just how color rotation works.

While with Tint everything becomes gray with a tint applied on top, with Hue Shift all color information is preserved, but shifted, which can sometimes produce unexpected results (which is why you can always chose to use one method or the other).
Personally I much prefer the results provided by the new Hue Shift method. If you actually watch the video above, wallpaper hot-track colorization is all done in that video via the Hue Shift method.
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On other news, tomorrow I will be increasing the price of Nexus Ultimate back to the original price of USD $24.95. If you want to save $5, buy it now while you still can.