well its not really analog, you will be still digitally manipulating the leds, your eye / brain is the component smoothing that out to analog interpretation
(yea ok you could but ...)
what you need to do is have something that can take a weak current and modulate a larger current, this is a generic text book description of a transistor, course your typical transistor will only hold a handful of ma at best
so depending on how much current your calculations show is what kind of parts we can suggest (mosfet ssr etc)
least your not trying to control hundreds of rgb leds (so led count * 3) individually, that would GREATLY increase complexity