There are no absolute truths
Painting is definitely one of the most difficult things a person can attempt. There's so many unknowns and so many choices to make. I've personally been struggling with so many questions for a number of years now but I just had a revelation today about using my own judgement.
My latest revelation came up because I recently varnished a bunch of my paintings. After the varnish dried, I was looking over them and going back and forth between whether or not to put a second coat or not. I was being pulled back and forth until I realized something. I wasn't looking at each painting individually, I was thinking of it as an absolute whole.
I realized that some of the paintings needed a second coat and some of them didn't. So then you could ask yourself, how do you decide which ones need a second coat and which ones don't? That decision is made through your own judgement.
I looked at each painting individually and decided whether or not I liked the surface quality. I saw that some areas of the varnish were too sunken in and the other paintings had a nicely uniform surface. What I found is that each painting accepted the varnish a little differently. This could be affected by a ton of different factors from the humidity in the air to how much varnish I applied.
I realized this is also true of my color palette. When I begin a painting from now on, I'm going to use my judgement to decide what colors I need for that particular painting. This is revolutionary for me because I finally feel like I have some clarity after many years of confusion. An example of this is if I'm painting a field of grass, then I need greens, yellows, white, browns and maybe some blues, but I know I definitely don't need a bunch of reds or oranges.
This idea was reinforced after I saw a photograph of Sorolla painting in his garden with only about 5 colors on his palette. I realized that there is no standard palette for a professional artist. I really love Sorolla because he represents a direct sincerity and truth, rather than a focus on superfluous technique.
I wish there were an easy way to predict the results and an easy way to make decisions, but the truth is that there isn't. All you can do is keep working, keep testing out knew things and keep gaining experience.