Country of origin primarily
Manga refers to Japanese comics (many are drawn in black & white due to the maturity and customs of the publishing industry). While they were traditionally hand drawn (which played a big part of being black & white), many modern manga are now drawn using digital tools either fully or partially (this is in part due to the lack of jobs that can afford to pay assistants since piracy has hurt income streams)
Manhua refers to Chinese Comics. Due to when the publishing industry focused on exporting its comics like the US & Japan, most are digitally colored
Manhwa refers to Korean Comics. Like Chinese manhua, except due to South Korea's status as an official ally of the USA, has been able to benefit from US publishing practices to give themselves a competitive edge (which plays a small part in why Manhwa is generally seen as better in comparison to manhua)