Paul W. answered 05/04/19
Dedicated to Achieving Student Success in History, Government, Culture
So far as I understand it, colonization involves a combination of the seizing of foreign territory by a kingdom / empire / nation-state and the post-conquest settling of people from the Imperialist power in the conquered territory. However, there is almost always a distinction upheld between the Imperialist 'Mother Country' that has undertaken the project of colonization and it's colonies, which are are almost never geographically contiguous with the Imperialist 'Mother Country.' This distinguishes the obtaining of colonies - often located overseas from the Imperialist 'Mother Country' - from territorial expansion by a kingdom / empire / nation-state, territorial expansion that serves to increase the size of the kingdom / empire / nation-state in question.
The distinction between the Imperialist 'Mother Country' and said country's 'Colonies' is demonstrated by the contrast between the treatment of the territory the United States obtained in the Spanish-American War (1898). The United States seized the kingdom of Hawaii (which wasn't even a possession of Spain) which was incorporated into the Union that is the United States in 1959, thus becoming one more State in the Union, with the same rights accorded to all States. The same can be said for California (along with all of the rest of the territory taken from Mexico as a result of the U.S.-Mexican War).
By contrast, the Philippines, which was a Spanish colony, was taken over by the United States, but was never incorporated into the Union as a State. It remained a colonial possession of the United States until it was given its independence in 1946 (Puerto Rico, on the other hand, has and continues to occupy a middle ground as a territory - neither colony nor State).
To be sure, citizens from the United States settled in California during the time in which it was part of the newly independent nation-state of Mexico and, of course, in even greater numbers after the United States had obtained California as a result of the Treaty ending the U.S.-Mexican War (due to the Gold Rush). But 'settlement' is not the same as 'colonization' when the territory being settled is not a colonial possession.
Drawing a distinction between a territory in which U.S. citizens settle that is destined to become part of the United States and a colony in which U.S. citizens settle that is not destined to become a part of the United States should not be interpreted as condoning the acquisition by the United States of California or any other territory through wars of conquest. In other words, distinguishing between settlement and colonization is not intended to be a value judgement.