This is a before - after Screenshot Collection I made to calibrate my eyes in the early days of Core Mining...
Note that lighting conditions have a
profound effect on the specific color you're looking for from a distance.
Only when fairly close-up, the trick of maintaining the Pulse Wave Scanner depressed revealing a 1 sec inverted color-scheme will give a very reliable indicator if an Asteroid is truly a Core Asteroid.
The coloring can change when closing in to inside ~3-4km, then again when approaching it to inside ~1.5km; thus not everything that looks very promising from a distance is good - but the opposite is also true.
Average looking ones from a distance can become ultra-bright inside 4-5km under some lighting conditions.
Also note that the smallest type of Asteroids
never seem to be a Core Asteroid - presumably because there's no destruction/explosion modeling for these in the 1st place, ruling them out.
Additional trick : when well inside the dust area of a blown-up Asteroid, any nearby Core Asteroid is often very easy to spot when Pulse Scanning from that position, if one exists. Doesn't always work, but
can sometimes result in a "lucky streak".
PS. I like entering Hotspots with 1000-3000km from the Center, if the Hotspot size permits it. Had a few well-known spots that didn't yield any Core Asteroid within 100km of the Hotspot center.
Just a theory, but approaching the Hotspot center from a larger distance so far tended to produce a nice yield, presumably/possibly because noone ever mined in that exact position.