What color display profile is set on your iMac?
Also, what is the year of your iMac?
For color calibration and management, we're currently investigating what can be done.
We want to make sure to have quality solution for all our users, and color management has been limited for iPad.
Astropad uses sRGB color profile. We currently support Apple ColorSync,
but an issue is that iPad screens are not calibrated to begin with.
The new iPad Pro model, has open more possibilities on improving color, but this will take time.
It's a top priority, and we're hoping to have a good fix in the near future.