I didn't see a definitation of calico.
For me calico is an abandonded term. Back in the 70's and 80's it was used to describe a fabric with small floral or geometric prints rather than the bolder designs. I still think of it as being a design reproduced from an earlier time. Homespun is a woven fabric usualy associated with either solids or plaids or strips. Threads are dyed first and then woven rather than printed 9on the fabric. It is available in varying weights. Quilters prefer the more tightly woven types as they do not fray. Also they are associated with more primitative type designs. Printed homespun uses the same base cloth but is dyed and printed with designs.
There have been several descriptions of muslin. It is like the unprinted cotton fabric and comes in unbleached or natural color or bleached white. Although there have been colors of soft pastels which were great to work with as it gave an old fashioned look. There is another fabric that often is confused with muslin. It is called Ecology Cloth. Is great for quilts and looks a little like unbleached muslin but doesn't have the brown flecks in it.
the brown flecks in muslin comes from the cotton in which all the seed residue was not removed.