Good question: Green are cargo ships. Red are tankers [both...

Good question: Green are cargo ships. Red are tankers [both liquids and LPG]. Dark blue are passenger ships. Aqua blue are tugs and special craft [e.g. pilot vessels]. Yellow are high speed vessels. Purple are pleasure craft. A salmon colour is a commercial fishing boat. If the system can't determine what category the vessel is, it displays that vessel as white.
Anything that looks like an arrow is moving in that direction. The bigger/wider the arrow, the larger the vessels is. A circle represents a vessel that is moored or anchored.
In the above snapshot, I removed fishing boats and pleasure craft from the map display to make it less complex.