If I want to go to Mexico for 30 days, then move on to the USA, would my VWP days be 90, or 60 (-30 days for the time spent in the "connected country")? I imagine its 90 days because they should start counting from ingress to the USA?
I understand that my 90 days are not reset if I don't leave to Canada or Mexico, but I'm confused about the other way. Imagine I went from Colombia => Mexico => USA, would that basically optimize my available time in the USA?
Your supposition is correct (you'd get 90 days in USA). The details are here but it may be simpler to consider the reason for the inclusion of time in Mexico (for example) being deemed to count towards the limit, when so deemed. The US wants to discourage visa runs, by making them expensive (ie long-distance). That puts an obstacle to maintaining a job in USA by someone who may not legally perform it.
The risk of such illegal work is not really a factor of the origin of the travel to USA of a VWP-eligible individual (eg Colombia or Mexico). Any further visit to USA shortly thereafter will be assessed on its own merits ("more strenuous" - as mentioned in the first link), separate from the assessment made for a first visit or one much earlier. The US takes the risk a VWP-eligible individual might work illegally for up to 90 days at a time but makes it difficult for such work to be a series of periods of up to 90 days each with only short intervals between each period.
Colombia => USA => Mexico should also give you up to 90 days in USA. It is USA featuring twice in such a string when the allowance in USA itself may be less than 90 days in total.
