There's a whole bunch of different methods for calculating the Lightness/Brightness/Value/Intensity/Luma...or even define them, and I don't think you can be 100% sure what method a program has assigned to that factor, whatever the character. It's a bit of a mess I believe.

Wikipedia on HSL-HSV

Personally, in my Grafx2 scripts; I use HSL where L = Lightness and L = (max(R,G,B) + min(R,G,B)) / 2, as well do the program I think. But note that changing Hue is a "destructive" operation, as it may offset the Brightness/Lightness of a given color.
Example of Hue-tweaking problem (Check Palette/Adjust/HueLightness if it doesn't jump to the right place)
That's why I for quality operations use what I call HSB, but here B is real "Perceptual Brightness", rather than just another term for Value (yes, I've made it even more ambiguous :/). With this I can provide the option to maintain the Brightness when you tweak the Hue (as Green is brighter then Red is brighter than Blue, things normally get screwed up when you change the Hue of a Palette/Image without Brightness correction).

Now, I'm sure that this did very little to clarify things for you. It's just so confusing that I have to use opportunites like this to remind myself of the basics, haha!

And, yeah. Don't use Paint. Ever.

Edited by DawnBringer - 19 September 2012 at 8:01pm