Sorry @pkr1979 and @01101111 this is not a FOSS solution but you may be able to install Serif PhotoPlus under wine and use as a free workaround as I use my old paid £20 for PhotoPlus9 now on Trisquel which is based on ubuntu 16.04
When you create a new image, you can choose to work in different colour modes, i.e., RGB, Lab, or Greyscale, in either 8- or 16-bits/channel. Use a Bit Depth of 16 bit for higher levels of image detail.
Even Ye Old 2003 Serif PhotoPlus9 uses 24 bit
Bit depth in PhotoPlus9
*One of the main differences between PhotoPlus and most other paint programs is that we’ve put aside the restrictive notion of working with a limited number of colors. You can work on any image in 24-bit mode, accessing the full color spectrum via the Color tab. Native format (.SPP) images are stored in this mode. When and if the time comes to save in a different format, and reduce colors, PhotoPlus provides the Export Optimizer for maximum quality control. *
While novices will appreciate the ease of use this approach brings, more experienced users may at first need to adjust to the absence of color swatches and the constraints of working in 256-color mode. Still, we’re confident that the benefits of concentrating on image production first, and color reduction last, will soon become apparent!
Hope this helps?.. or could admin delete this post