Yes, I am not a professional painter, not even a student. I’m an amateur. Therefore, let my primitive question not surprise you. I follow some online lessons to paint nature and botanical items. They alway say something like, ‘Now take a bit of opera rose. The mix should be very watery. We will put some cobalt violet on top of this layer when it dries.’ How do they know which colors to use underneath? What you see with your eyes in reality is a violet flower with some white speckles. However, it is not simply violet. It has at least four unseen undertones you need to take into account when painting. How do I or how do you find out what they are? Please do not laugh☺️😳😳. You probably think I am strange, but I really do not know. Thanks in advance!
You have to detect the colours that are going to be used before starting the painting. If you can’t remember them then keep notes in order to have some kind of guide for the mixes you are going to need. There is also always a possibility to run out of your premixed colour and need to re mix it or to need it for making corrections etc. So better write down what you intend to use or what you have already used.
First detect the next darker in value colour after the white of the paper ( that you can preserve either with masking fluid or by avoiding painting over some areas).
If you paint leafs f.e then check where it might look yellowish or beige or light green. Keep some notes to remember where it goes what and then apply these layers of colours. Then you go to the next darker colour. Keep again notes in order to remember where goes which colour and then apply the next layer.
Built your painting by applying layer after layer going always from the lightest to the darkest ones.
When you paint florals: Detect what is the lighter of the -let’s say -rose or reds. If you are not sure about what you see, then use the Gimp’s colour detector. It will give you some guidance regarding what colour you have to use. It is possible to find some unexpected colour hues in some flowers for example an orange shadow on a pink rose. Add these too at the proper value of course. The colour variations on floral painting give life to the flower you paint and makes it more realistic. Watercolours are transparent so the colours you used at your first layers will be slightly visible under the rest of the layers.
I hope I helped a bit.. if you have any queries don’t hesitate to ask me.