[ver 1.10.106.0]
Quite certain this is a supplied color in the Custom theme: 446677
It is an off-blue color (a teal, almost), both visually and according to the web; its RGB is 68, 102, 119. As you can see, it's predominantly blue, with similar amount of greens but low reds; technically not gray.

As I wanted to make another swatch with a slightly darker shade of it, I've found I'm completely unable.

Entering in its hex, and pressing Enter, claims it to be a Monotone shade of black, even displaying the black-to-white slider.
The swatch still depicts as the correct off-blue color, and it correctly paints in that off-blue color.
However, when I go to the slider bar to try and change its shade (luminosity?) it immediately switches the swatch over to the Gray that I mentioned the slider shows. Specifically, it thinks the color is 5D5D5D.
(Note: It immediately switches even if all I do is click on the shade slider; it will in turn select the next higher or lower shade of Gray if I do move it)

I have even tried to convert different swatches, thinking perhaps it was that specific one that was bugged. Though, even the ORIGINAL swatch that is 446677 also shows the same Monotone slider.

STEPS TO REPRODUCE

1) Go into the Designer
2) Open the Paint menu
3) Go to the Custom theme
4a) Go to the bottom row, middle swatch, and click.
4b) Or, go to any swatch and input a Code of 446677, and press Enter
5) Witness the slider is on the far left in the Color Picker, and that its Shade Slider is at roughly 1/3rd from the left.
6) Click on the selected rectangle of the Shade slider (you do not need to move it); witness the visual color of the swatch and the Code have both changed, even though you've technically not made any changes...