I guess it would depend on the purpose of the meeting. If you are making a group project or doing a swap, I would define "muddy" as anything that is brown, grey or black ... or a combination thereof.
If it's a color theory class, I would look for fabrics that combine complementary colors (red & blue-green, orange & indigo-blue, yellow & indigo-violet or green & red-violet). Any of those combinations will result in a greyish "muddy" look -- especially when both colors have similar saturation of color.
That said, I probably overthink these things. I'm sure any fabric that generally reminds you of mud will be just perfect.
