You might consider the difference between Premium Muslin vs. Regular Muslin. The thread count being the main difference, the Premium is finer, smoother, and presents a more sophisticated end product. Regular muslin works well with feedsacks (and reproduction prints), homespuns (and their new version called the 'wovens'), and 1930s prints. It is the color difference between bleached and unbleached; whether your color scheme works better with an offwhite or darker tone. Here's a description of first quality Roc-lon:
Superior Thread Count, Superior Finish
78x78 Thread Count
Weight: 3.4 oz.
Preshrunk
In contrast, "Premium" often has a thread count of 128 x 128.
Better to buy a bolt of each, eh? Oh, and by the way, there is DARK Tea-dyed; MEDIUM Tea-dyed; and some mfg. have created one called 'Antique' (??)