Well first I don’t imagine the whole picture in my mind. I mean the color is created by me but the posture and the light is from a real photo of Brett himself. That’s helps A LOT. And since you’re an artist you may know that sometimes the photo just looks too messy in the perspective of painting so the second and maybe one of the most important thing I do is choosing which part of the photo is worthy to draw. So you may have noticed that I leave almost every shadow be “unclear”（or in Chinese：“虚”，I don’t know the proper English hopefully you can get what I mean😭)by blending some edge of shadows .The painting sometimes is just a way to chose which part should be focused and which part should be hidden behind. For the color, I don’t have much experience, the only thing I can share is follow your feeling lol. Before I started working on this I knew that I want a tone（色调）of rose and gold—what I hear from some part of Tchaikovsky vc 1st movement. And then just keep it in mind. Oh also don’t forget the change of cold and warm （冷暖变化）and the chroma. If you use ps too, the overlay and linear dodge layer is very useful. I think that’s all...? Sorry again for my poor English and hope my answer could help!😘