Change the plots in 'grains' into binary

[Jean-Du]

Is your feature request related to a problem? Please describe. The image output in the 'grains' directories are in colour gradient instead of binary, which results in grains at the top of the image being hard to see.

Describe the solution you'd like Change the image output in 'grains' from colour gradient to binary.

Describe alternatives you've considered A clear and concise description of any alternative solutions or features you've considered.

Additional context This can be addressed alongside #138