How to simplify when painting an apple...

If you want render the illusion of an apple I recommend you to concentrate on the volume and it's shiny texture (or not) before you start thinking of any details. You do so by painting (or drawing) the shadows, with them you get the light. If you have poor light source the vision of the shadows can be difficult. Change the light then, if you can, or if not, squint to see the darks. The more experience in drawing and observation you have the easier it is to see and "read" what you have in front of you.
If the apple is dark red for example tend to lighten the red on the light side. It's a little tricky to darken a red so don't start too red in the light (I've seen a few examples at my students tries) or you hardly get any contrast when you get into the darks.
You could use a Windsor Red as a base that you lighten with water and a little yellow at some parts (accept color changes i watercolor, it gives variety, rather than doing a well done and uniform surface of one light color.) I tend to use some Permanent Rose too into the mixes in some parts of the light. To make a darker red I use blue lately in my mix with the red, as I see purple in my apple's shadows.
The most important to keep in mind is to forget about details like color changes or spots on the skin etc. It can be hard to see the difference between light changes and skin color changes. Learn to do a simple apple in volume before learning to a more realistic one.
The process of a more developed apple is the same but adding details. Attention : Use details with care (!), express only those you think you need. You don't want to "tell" everything, or the result will be boring, as you give no chance to the the person looking at it to you use his imagination.
The more you practice the easier it gets, until you don't think of your difficulties any longer, simply because they are assimilated.
It is process that should be applied for most subjects in my opinion.

When I paint I don't use rules. I have to search for explanations for my students to help them as much as possible to start, but I try to make them understand that they have to learn how to observe and render volume, mix colors etc. by themselves with practice.
When I paint for myself I observe and I try to paint what I think I have seen, and the effect I want to give of it. All colors change in every situation; in relation to nearby colors and the light settings. Rules couldn't possibly be applied really.