I select the shade of color on a gradient fill. A dark blue = a dark red, a light blue = a light red. Rinse and Repeat.
If you know that the colors may change on an illustration like this, a trick in Illustrator is to to use spot colors to build the components of the image. For example, the car body could be shades of PMS 485 red. In that way, the color can be changed in the palette rather than swapping out individual colors within the illustration.
To build the shadows and darker areas, a gray scale transparency is built over the areas that one would want to affect. (I think something similar is under the lens effect in Corel.) Adjusting the levels of transparency on the individual vector parts creates the dimension, and still allows for a super quick change to the base color from the palette menu.
This technique is helpful when a shadow or reflection crosses an area of two colors - like the trees and buildings you mentioned in an earlier post. A transparent vector shape of that could be drawn to overlay both the red of the car body and the white of the stripe. With the single transparent overlay, one doesn't need to adjust the two base colors to create the same shadow effect.
Hopefully that makes sense...
It does lend itself to a more illustrative look than using different tones of the same color. But, if you vary the colors and the levels of transparency in the vector overlays, it helps to keep the image from looking too flat. ....and it can be a bit of a memory pig.
Great Illustrations! BTW
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