Few things in the hope it can help 
Maybe telling people in the documentation where to find it in the GIMP's menu would be a plus (I needed to look at the code to find out where it is in the menu)
Filters ➤ Render ➤ Fill cells with random colors is not in the doc.
It rounds my hexagons (not keeping them sharp/straight)
A .xcf with my tests if you want to take a look it
Untitled.xcf.7z (Size: 481.78 KB / Downloads: 159)
I did tried
Alpha to selection/Invert ➤ No grow ➤ No bleed
Alpha to selection/Invert ➤ grow +1 ➤ No bleed
Alpha to selection/Invert ➤ No grow ➤ bleed +1
Alpha to selection/Invert ➤ grow +1 ➤ bleed +1
All of the above tests did round more or less the hexagons in a way or another
1000 colors... in the example below I have hard time to perceive the 330+ different colors, merely 50 or 100
Similar range may be to near to each other to be perceptible by human? Or maybe it's just me...
Maybe telling people in the documentation where to find it in the GIMP's menu would be a plus (I needed to look at the code to find out where it is in the menu)
Filters ➤ Render ➤ Fill cells with random colors is not in the doc.
It rounds my hexagons (not keeping them sharp/straight)
A .xcf with my tests if you want to take a look it
Untitled.xcf.7z (Size: 481.78 KB / Downloads: 159)
I did tried
Alpha to selection/Invert ➤ No grow ➤ No bleed
Alpha to selection/Invert ➤ grow +1 ➤ No bleed
Alpha to selection/Invert ➤ No grow ➤ bleed +1
Alpha to selection/Invert ➤ grow +1 ➤ bleed +1
All of the above tests did round more or less the hexagons in a way or another
1000 colors... in the example below I have hard time to perceive the 330+ different colors, merely 50 or 100
Similar range may be to near to each other to be perceptible by human? Or maybe it's just me...