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.