GrabCad Voxel Printing on J750 - how to assign empty space?

Hi All,

I have managed to voxel-print a scientific dataset by using my own script to convert the raw data into a series of png images accepted by the GrabCad Voxel Print utility.
Each pixel in the png images is coloured according to one of the six inks available, and the assignment made accordingly in the voxel print utility, as per this guide:
https://help.grabcad.com/article/230-guide-to-voxel-printing?locale=en

I have attached a png image from a single layer as an example.

Here is the resulting print:
https://www.youtube.com/watch?v=aVR9SQz9j0I
https://www.youtube.com/watch?v=eQX9HNwIY3s

Now for the question:
- If I want to print a different shape, e.g. a sphere, or a cube with a corner cut-away, how would I specify the empty space using the png image method?
- Do I need a 7th colour in the png images? I am currently using the alpha channel to represent VeroClear (transparent) resin, but maybe I could represent VeroClear using a different colour, and use the alpha channel to represent empty space? But still, how do I make this assignment?
- Am I correct in thinking the printer will automatically fill this with support resin? (which is later dissolved in water)

Please note that I don't have access to the printer or GrabCad voxel print utility - I just send the png files to someone who does have access, along with an annotated screenshot of the above guide, to show the colour mappings and settings. (this person cannot help with my query).

Thanks in advance,

Stuart

Edit: if you click on the attached png image below, the alpha channel appears as black