When I was learning to paint icons with Aidan Hart he gave us the principle of learning to paint in a particular style: 'copy with understanding'. This is true regardless of the style we wish to learn. For example, I am now focussing in the classes I teach on 13th century gothic images of the School of St Albans. I use the same principle: I look carefully, I try to understand what the artist was aiming at and then I copy. Even the great masters get things wrong and occasionally you can errors. On these occasions we don't copy the errors but try to have in mind the ideal that the artist was aiming for and correct what is there. In order to be able to make such a judgement you either need to be very knowledgeable about the tradition, or have a good teacher who can point such things out to you. The motto that Aidan used was 'think twice and paint once'. In other words, study carefully and think before you paint.
One thing that is sometimes very difficult to ascertain is how the colour and tone effects of a painting have been achieved. Usually what we are seeing are the combined effects of multiple transparent washes and glazes of all sorts of different tones and colours. To help us, Aidan encouraged us to look at old mosaics. The reason for this is that you can see how, for example, a flesh tone has been mixed because each constituent colour is present as a pure-coloured tessera.
Also, I can see more clearly than in a painting devices that artists use to describe form. For example, if we look at the mosaic of St Apollonarius below then we can see that the mosaicist has used all the devices that a painter is told to use on the face, but they are more obvious. So the line defining the upper lid is darker than that defining the lower eyelid. There is a dark red line between this and the eyebrow, which is the line where the eyeball goes back into the socket of the skull. There is a red line that defines the deep shadow between the hairline and the brow. Similarly, there is a red line below the end of the nose. All of these things are present in painted faces, but often they will be translucent to some degree and so the effect is more subtle and this makes it more difficult to discern what the artist did.
This mosaic, by the way was in the reliquary of a monastery that I visited recently in the US. I was told that it came originally from Ravenna! Whatever the details, it is a great piece of work.