Everybody knows that the best ice cream is, objectively, marlin. Sheesh.
Obviously you’ve never tried garlic ice cream. I haven’t, either, but I do hear there’s such a thing.
Back to the main question, personal tastes are just that: personal. They vary from peroson to person in innumerable ways. There’s no such thing as a most rational color for your house (except for some exceptions, like using light colors in warm, sunny climates to prevent the absorption of too much heat; but even then if you can afford the utility bill, you could rationally paint your house black).
As has been mentioned, you should be able to know why you pick a particular color. Maybe it blends well with the surroundings, maybe it makes the hosue stand out from the surroundings, maybe it’s your favorite color, maybe it was the cheapest paint you could find and you don’t care because you don’t look at the facade of your house very often. These are all valid reasons.
As to a career, that is more serious, but equally wide open. You should pick a productive field of endeavor. Past that, which field you choose depends on your personal interests and goals. If you like medicine and want to find a cure for a given disease, then maybe you’d be best off studying molecular biology, but maybe also studying medicine and working on research rather than treatment afterwards. If you like to fly and wish to help defend your country, then clearly you should try to join a branch of the armed forces (all branches in the American military operate combat and combat-support aircraft). If you like to play football and want to earn a lot of money, you should try to become an NFL pro.
Molecular biology is no more nor less rational a achoice than medicine than Air Force fighter pilot. Except if the choice conflicts with your interests and goals. If you want to research cancer, you won’t do it flying F-15s over Iran, or playing tight end for the Steelers. That would be an irrational choice given your interests and goals.
As to ice cream flavors, or any other sort of culinary choices, there are no absolute standards for flavor. There are standards for what you should eat and in what amounts, but not on what it should taste like. The general rational rule would be to eat such things as you find enjoyable in proportions that are good for you (no single food is entirely good or bad nutriotionally, but rather depends on how much of it you ingest). So for example you shouldn’t eat burgers at every meal, but a burger every now and then is ok. You shouldn’t eat only lettuce, either, but you can have a salad or two with diner and/or lunch every day. And so on.