The idea is to duplicate the image on different layers and use different opacities for the colors. The lighter color layers are set on top so that the darker colors (layers) show through. It involves a lot of work.
basic art technique called shading. it gives an object depth and dimension. easier to do it by hand on paper than to do it on the computer. mask the area not to be painted and use the airbrush tool in photoshop or the brush tool in illustrator. separate layers for each color as stated above.
Given that you are working in vector, you need to design for the garment color in mind. Create a custom color that will be separated as a blend of two different colors, or just simply drop out the base for the 2-tone effect if it's going on a dark shirt. You'll have to figure out what works best for you and it does take practice. Good luck and let me know if you have any more questions.
it takes alot of practice and the right tools. depending on your final goal as an artist (just production work or illustration too) i would suggest getting photoshop (illustrator you have) and a wacom tablet.
here's 2 tuts i found that kinda explain the shading techniques in photoshop:
I don't know if this helps, but here is a step by step example of how I would create that look;
Draw the wings (taking the blue section as the example) as vectors in illustrator, each feather has a black outline on the dark blue base layer even (though it looks like it has no outline because of the black background - the outline is set using one of the art brush options to give it it's style)
....then on another layer add the medium toned shapes (they are probably a lighter blue set to a low opacity or a white set to an even lower opacity)
...then on another layer draw the highlights again using white at a low opacity ...
The only problem you may have is that there is actually a very subtle gradient applied to it helping give the image shape and form. There also seems to be a slight (maybe gaussion?) blur applied to the layers (notice that the lines aren't sharp?) As I'm new to tshirt designing I'm not sure how much of an issue this would be in the printing process? (It would help me a tonne to know)...
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