If Color3 values had h, s, and v properties (not sure why they don’t when we have a HSV constructor), you’d be able to do
color = Color3.fromHSV(color.h, color.s, color.v + delta)
Value in HSV does exactly what you’re wanting it to do (control brightness/darkness). I think completing an existing API is a better bet than adding the new concept of alpha.
Edit: Posted feature requests here Add h,s,v properties to Color3 values
Edit2: Apparently you can do
local h,s,v = Color3.toHSV(color)
color = Color3.fromHSV(h,s,v + delta)