acepilot, for instance, my latest Fw190D-9 skin uses nearly 40 layers. It's a long boring list, but to help you get the general idea of why skinners use layers, I'll put them down here from top to bottom:
Weather: Smudges
Weather: Highlights
Weather: Scratches
Elevator tab
Aileron tab
Text:"Steckdose für..."
(L)W.Nr.211016
(R)W.Nr.211016
Text:"Fach für Erste.."
(L)Indicator zum Anbringen des Ruderschloss
(R)Indicator zum Anbringen des Ruderschloss
Text:"Anzeigegerät"
Text: (R)"Reifendruck 5 atü"
Text: (L)"Reifendruck 5 atü"
Text: (R)"Hier aufholen"
Text: (L)"Hier aufholen"
C3Fueltab fwd tank
C3Fuelrab rear tank
MW50tab
Tail ruffles
Wing ruffles
Rivtes
Panel lines
Cowling Emphasis(R)
Cowling Emphasis(L)
Original Panel lines
Testsheet Layer
Jutta
Balkenkreuz underwing
(L)14 < + -
(R)14 < + -
(L)Priller's mark
(R)Priller's mark
Background
Among all of these layers, all you really need to make a complete plane skin, is just one: the "Background". The background contains the main colors.
The rest 33 layers determine how detailed my plane will look.
Imagine that each of these layers like a transparent film. When you draw a little bit of something on the layers, and then stack them up all together, it becomes a complete picture.
When you see something wrong, you just pick out the layer that contains the part which needs to be fixed.
When you see something missing, you just insert a new layer containing what needs to be added, in the appropriate place.
When you see something should be deleted, you just pick out the layer that contains it, and erase it without effecting other parts of the plane.
..
If you've seen my thread on Priller's Fw190D-9, Wmaker pointed out to me the underwing cross was in the wrong form. It took exactly about 15 seconds for me to correct the mistake. However, If I had drawn the cross directly on the background, I would have had to erase the whole thing carefully, draw in a new cross, fill out the spaces that don't fit, see how the color matches and stuff. Probably more than 10 minutes to do the work.
Layers are all about detail and efficiency. Not all the skinners who use layers are good skinners, but all the skinners who are good are skinners who know how to use layers.
...
When all the work with layers are finished, the finalized BMP is something like taking a snapshot of all the stacked films - the layers don't go into the BMP, only what the whole, stacked layer looks like, becomes saved in the BMP.
Thus, you have to change the RGB format file with numerous layers, into an "indexed color" format. During the process, you will lose some color quality, so skinning must take that into account, too.
