I took the liberty of rummaging through the files and identifying which planets in the game are designated mining colonies. I feel this is important to post since afaik the game itself does not directly tell you this information (without inspecting your income on certain planets). For those who don't know, normally an economic building such as a Mining Facility or Tax Collection Agency provides income equal to twice the base income of the planet its built on. Mining Facilities constructed on planets that are Mining Colonies will instead provide 3x the planet's base income, or an increase of 50% over a non-mining colony of equal value. Note that I specifically said Mining Facilities - Tax Collection Agencies do not benefit from being built on Mining Colonies so most Imperial factions do not benefit from this mechanic.
The list of mining colonies is as follows:
Bespin
Bilbringi
Biniir
Bonadan
Dathomir
Dolomar
Dubrillion
Halanit
Hast
Hewett
Kampe
Kessel
Muunilinst
Nkllon
Pardron
Rishi
Sullust
Trasi
Vondarc
Wroona
YagDhul
Ylesia