Both Log and Int are not entirely purple trades. There are jobs within those occupations that are environmental, and jobs that are purple. For either, you do a common component to the training, then an environmental component.
For example, as an Army Log O, expect the first posting to be to an Army unit (Service Battalion) not to Esquimalt (Sea Log), Trenton (Air Log) or Ottawa (mostly "purple"). Sea Log, Army Log and Air Log are different sub occupations within the Log Occupation.
If you are a Sea Log O, and then apply to become an Int O, you will be considered against what the intake for Sea Int is that year, since the experience you bring in your previous trade is part of the criteria. If there are only openings in Air and Land Int, and you are Sea, you won't get an offer (happened to a friend of mine). But that needs a crystal ball...since you don't know how many positions in any environment are available in a future year.
So, it matters, but only from the perspective of you'll work mostly in that environment at the outset of your career. If you hate the idea of working on a ship, Sea would not be a good choice. If Petawawa, Shilo and Gagetown make you shudder, think long and hard before you accept Army. Keeping in mind that any posting is possible, but with the suboccupation you end up in, some are more likely than others.
Some occupations, uniform colour is just that, the colour of the clothes in your closet (ex - I'm a Training Development Officer, makes no difference to my career that I wear an Army Uniform. My next posting could just as likely be to a Navy organization as to an Army one). Others, like Log and Int, there are environmental aspects within it, so it does matter to a degree. MPO (your 3rd choice) - not sure about that one. I don't know how "purple" that occupation is.