In the VSE, there's color balance, curves, hue correction, mask and Brightness/contrast as strip modifiers. But saturation is a normal value of the strip. This makes it impossible to apply saturation via masks. Is there a plugin that fixes that or a way to add saturation as a modifier or a apply a mask to it some other way? Kind of annoying. it's not preventing me from achieving anything great or important, but I'm sure it's more pressing to someone!
Use a Hue Correct modifier to the strip, this modifier can use a mask and lets you adjust the hue, saturation and/or value of the strip.
By default the modifier has nine points across the spectrum so you can adjust specific colours, by selecting each internal point and clicking the
X just above the curve editor you can have two points to get an even adjustment through the hues.