From the FAQ:
"The colour of the callsigns indicates the source of the information for a given report:
Black (like XX0XX) are spots from the DX-Cluster
Blue (Like XX0XX) are spots coming from the WSPR network
Green (Like XX0XX) are reports from the Reverse Beacon network
Magenta (Like XX0XX) are reports from the PSK-Reporter network and FM-SWL reports coming from FMList
Brown (Like XX0XX) are AIS, APRS, WSJT-X and MAP65 reports sent using AIS DX Aggregator, APRS DX Aggregator, WSJT DX Aggregator and MAP65 DX Aggregator
Violet (Like XX0XX) are QSO taken from the on-line station log"
https://www.dxmaps.com/spots/faq.html#04