After I learnt how to draw, I started painting and began to learn that as well! You have a brush, a support and paint, simple.
However it is with paint where the real challenges start, not so much the paint (if it is good quality) but it's critical, inherent quality; colour.
So many beautiful, rich and powerful colours, so many subtle, neutral, deep ones...
At the start, you pick a subject and begin the process, and the almost infinite variety of colour blows you away, your focus is on colour and trying to represent the colour correctly. This is very distracting and in fact I now believe completely secondary to other factors in painting.
It is important to understand human visual perception will make huge allowances for inconsistencies in colour if values and temperature are correct. If these two factors are right, the colours can be way off the mark but you will perceive a realistic rendering of form and light.
Trying to crack realistic painting through pure observation of colour is in my opinion impossible, it needs to be broken down into component parts.
By restricting the palette to a few colours, maybe 4 or 5 (roughly the primaries), you have to make these colours really work and the knowledge gained in how they interact with each other in a painting and their breadth of range is amazing. You learn about the power of colour temperature in a picture, this is the real source of light in a work, temperature relationships. Simplflying it greatly; a sliding scale of cool and transparent for shadow to warm and opaque for light.
Beneath this stage of construction is value, the transition from light to dark, first the eye must become accustomed to seeing and distinguishing these values. A very good way is to go back to a monotone drawing of the subject. Good composition is totally based on well arranged value and line.
Through focusing on first, value relationships and then second, temperature relationships, realistic colour in painting is solved and you will be no longer overwhelmed or distracted by the array of colours out there. However maybe most importantly it is an incredibly fascinating way of looking at the world around us and brings a new delight to the things we see every day.
I suppose the message is simplify, the true purpose of painting; break down nature into it's component parts and find the 'truth' within them.
The Cezanne painting at the beginning of this post is a visual example of these ideas taken to the highest level of simplicity and skill. When you look really look at the colours in the painting they are unreal, they have too much chroma, however the values and temperature are spot on and this gives an image which is almost more real than reality.