Pooface said:step 3, matching up colours to the image
Actually, somewhere around here there're links to several sites that have generally accepted RGB values for the standard colors of various nationalities. So it's just using those values for the RAF colors on the skin you want to do.
step 4, would be starting aon a base layer? now this is where im stuck (i havent started yet btw) how will i know where to be putting the colours? if i put them in the wrong place they'll be in the wrong place on the plane. is there any way to know where to put the textures?
What you should first do is mark out the structure: panel lines, rivets, etc. Once you get these down, THEN worry about the paint. Here's the steps I recommend:
1. Create a new 1024x1024 file with a plain white background.
2. Import the stock skin (resized to 1024 if necessary) as the next layer above this.
3. Create a new layer above the original skin where you just roughly trace the outlines of all the parts of the plane in some bold color like pure yellow lines. This way, you'll keep from getting upper wing paint on the lower wing, etc., without the original skin's colors confusing the issue. I recommend always leaving this layer as the uppermost as you proceed, and making it invisible except when you need to see where the edges of the parts are.
NOTE: There will probably be a bunch of little parts that you can't tell where they go right off. Color these in various bold, distinct colors and then save the skin with ONLY the outlines and white background layers visible, then look at it in the skin viewer and see where these colors show up on the plane. The put some text labels on the outlines layer to help you remember what the parts are.
4. Create another layer above the orginal skin and rough out all the panel lines. Again use a bold color, like pure blue. Use the lines on the stock skin as a guide, but EVERYBODY's panel lines are usually wrong in a few places, so draw your lines as your research tells you they should be. Save the skin with just this layer and the white background visible, and see if your lines are in the right place in the skin viewer, and line up correctly across joints between different parts of the plane. Tweak your blue lines as needed and repeat this step as often as necessary until you've got all your lines right.
5. The layer created in step 4 is just a template. Now you make a new layer on top of it for the actual lines you want on your finished skin. Just trace your blue lines with gray and black lines as needed, depending on what type of panel each line represents.
NOTE: It's best to create the real panel lines for each part of the plane separately on their own layers, or at least break them up with like both sides of the fuselage, and both upper wings, both lower wings, on their own layers.
6. Create new layers for the rivets. As above, use different layers for the rivets for different parts of the plane. The rivets need to be on a different layers from the panel lines because you'll be doing a LOT of erasing here, and you don't want to mess up the lines. What you do is draw gray lines where you want a row of rivets, then go back and erase gaps in them to leave 1x1 dots for the rivets.
7. Add further layers for things that fake 3D effects on flat poly areas. These would be like the curves at the fronts of the control surfaces, the stringers for fabric-covered areas, etc.
This pretty much completes the structural elements of the skin. Now you can get to the fun part, which is the paintjob

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ok, step 5 would be another layer of colours, but varying the texture, making sure it doesnt look flat. adding any nose art and decals?
I put my paint on top of the structural layers. Again, I use separate layers for each part of the plane. To see the structure through the paint, you have to set the opacity of the paint layers to like 85%, which has the added benefit of "thinning" the colors a bit, like you do with plastic kits, for a more realistic look.
I usually do the camo pattern colors for a given plane part on the same layer. Do it like you would a model, lighter color first. Use the airbrush tool with like 100% opacity, size 20, and 50% coverage. This means you have to do numerous passes over the same area to get a solid coat of paint, but you get some subtle variations so it really looks sprayed on, plus a random scatter of pixels with no paint at all, which look good like chips in the paint. Do the edges between colors with a smaller airbrush, like size 6, and work it more or less often to get the amount of feathering you want. Then select the edges between the colors and use some blurring option to really get the sprayed edge how you want.
On some planes, especially Germans, sometimes you need to do some subtle mottling effects. I usually do these areas on their own layer, distinct from the main paint layer, so I can vary the layer opacity separately, plus give it different blurring without blurring the mottles into the underlying color. This makes them look sprayed on after the main colors, which was usually the case in real life.
Put the "decals" on 1 or more other layers. I usually have a layer for national insignia, another for unit markings, and another for stencils. Because you want the structure to show through the markings, you have to have that layer at like 85%, too. But this lets the paint show through the markings, so you usually have to go back and erase the paint under the markings. Or maybe not, depending on the plane you're doing. If your research shows the paint through the markings, leave it that way.
then weathering?
Yeah, weathering is the last step. You add separate layers for each type you want to do: exhaust stains, oil leaks, gun smoke, mud splatter, greasy handprints, muddy footprints, etc. You can also go back and erase small bits of the paint layer, to let the white background show through, then blur these "chips" a little, which gives a good "metal showing through" effect.