Great point, color perception does change over time! I think even the color switch between pink for girls and blue for boys happened not a long time ago too, in the 20th century, so it really makes me think about what will happen in the future.
Oh yes, the health bar really gets me conflicted, not only conflicted, but I even get a bit more philosophical about it. Why is it even red? Is it associated with the heart icons that represented how many lives a player has? Is it just because red is great at grabbing attention? Is it just somebody’s favorite color? Hmm, some red healing spell that now I can think of is health leeching from other enemies, they are usually blood red with some black accents. Yeah, it is harder to find the right spot, I tend to stick to the classics and observe what others do, as I think I am not there yet to break the rules. If all the VFX artists united we could start a revolution and change the so-called rules, hehe!!
The VFX readability over the backgrounds is one of the most common things that I struggle with, so you are not alone here. I mean ideally, you could have two types of effects: one for the bright background and one for the dark one (or just have some of the elements of the effect changed, for e.g. have different smoke) and switch them with scripting. Sometimes this is just way quicker and helps to keep you sane! If there is a constant day & night cycle… Well… That’s on you buddy A lot depends on the lighting in the scene, sometimes you can solve problems with lit particles, something I tend to use on steam/smokes in the daylight scenes. But yet, this is still a mystery to be solved (a very painful one too).