"Mid-tone" is such a hard term to decipher. It can mean just about anything. Here's my completely unscientific thought process: The "Fresh Amour" mattene that came out with Posh Paradise (here are Temptalia's pics: http://www.temptalia.com/mac-posh-paradise-mattenes-swatches-photos-reviews#more-31571) was described as "light lavender". I'd expect that something described as "mid-tone" would be noticeably darker. That probably means it would be a bit deeper than LW, but might be close.
Of course, it's not like there's some empirical way to evaluate "light" vs. "Mid-Tone", but anyway...
I suppose if you wanted to get really technical, "mid-tone" in photography refers to luminosity - the midpoint between pitch black darkness and total light reflectiveness. In paint mixing, it'd mean a pure base color mixed with white and black pigment, to reach a value that is right at the halfway point of brightness and darkness (somewhere in the area of 50/50 white and black mixed in, but this would depend on the starting pigment's hue). Whether MAC takes this super literally or not isn't something I'd know for sure. Mid-tone would certainly be different than "dirty", which usually implies a color that has its compliment mixed in to muddy the hue, which would be separate from how much white or black it's got, which will mostly be affecting the lightness. For example, this would be the difference between mixing white, black and brown to get light tan or dark chocolate; and mixing in a little extra green into the brown to get a taupe or clay color, separate from how much white or black you have.
Here's another example: Go For It lipstick is described as "mid-tone blue purple" and as all us purple-holics know, it's a color that isn't milky, but it's not murky or blackened either - just a color that reads as clear but without making your lips seem like they're too vibrantly or brightly standing out from the rest of your skin tone, no matter if you're in bright light or shade.
I don't own Lavender Whip, but looking at swatches and the official MAC description, I'm going to guess that it's brighter than what Seasoned Plum will be, if not lighter. Think Lavender Whip but standing out less on the lips when applied and we may be getting in the zone.
