Hmm. I'm sure others may have better ideas as I haven't designed any shirts and therefore may not be aware of all of the colors available.... that being said, maybe a burgundy/maroonish/dark red or a shade of darker green. I can't quite picture what the dark green would look like with the brighter colors of blue that you used, but it would be interesting to see how it looked.
Those two, and maybe Dakota Slate. That's all I can think of for right now..... hopefully other people will chime in with suggestions. :o)
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