When I think of "matchy-matchy" I think of Garanimals and DTM (dyed to match). Because of this, I also tend to associate "crayon box colors" as matchy-matchy moreso than a more subdued palette.
Matchy-matchy goes beyond coordinating colors. I think of it as "excessively/obsessively" matched. A woman with red shoes and a red handbag with a black dress is not matchy-matchy. Neither is a woman with a yellow scarf and yellow belt with jeans, a white tee, and cognac boots. This is particularly true if the "matched" pieces have texture, pattern, and/or a slight variation of color.
Now a woman in a sea foam green tee, cerulean blue pants, sea foam and cerulean braided belt, cerulean blue bag, sea foam green socks, cerulean blue canvas sneakers with sea foam green laces is obviously matchy-matchy. On the less extreme side, a woman in a dress or jeans and a tee is also matchy-matchy if her shoes, bag, belt, scarf, all match each other and/or a color in her dress or tee.
