Are you sure that was 100 mA and not 100 uA?
The aux LEDs are powered directly by MCU pins, and the MCU has a maximum power output of 40 mA. I measured a board with its resistors shorted out, and with all channels running at full power, it measured 42 mA. The Atmel reference manuals recommend against doing that though, because running that hot can cause damage long-term. But that was only a test; lights aren’t shipped that way. Usually it’s much lower – around 5 mA on high aux mode or 0.05 to 0.20 mA on low aux mode. People typically use low mode because high mode uses too much power.
If the goal was to provide active RGB functions while the light is “on”, like while a user is actively using it, I think a NeoPixel setup might be a good way to do it.
The aux LEDs are generally not used while the light is “on” though… instead, they emit a dim glow while the light is “off”. It’s like adding tritium vials to make the light easy to find in the dark, except they’re slightly brighter, don’t have a radioactive half-life, and can blink or change color.
Typically, the MCU measures voltage every 8 seconds, changes the aux LED color, and goes back to sleep to save power. It cannot do any I/O or PWM while it’s asleep… it just passively allows a trickle of power to flow.
Technically, it could PWM the RGB pins to adjust brightness or generate in-between colors, just like the NeoPixel does… but keeping the MCU awake enough to generate a PWM signal uses way more power than just leaving the LEDs on in sleep mode. So it would be self-defeating.
OTOH, there are some flashlights which are designed to use colored LEDs for primary output modes. I’m planning to work on some in the next couple weeks, even… and doing some big code changes to improve support for that. But for those, a NeoPixel isn’t bright enough. These colorful flashlights are measured in A, not mA. And the NeoPixel’s 8-bit resolution might not be enough; I frequently use 10 bits or even dynamic bit depth to make the low end of the ramp lower and more smooth.
The models I’m using for development are warm/cool/red and flood/throw/red, but they could also be white/red/blue or red/green/blue or white/infrared/ultraviolet or whatever. In addition to that, they have a lighted button and sometimes RGB aux LEDs. Would be funny to have RGB on both primary and auxiliary LEDs… but it’s an option, so I plan to support it. I’m adapting some of my lightsaber code for that purpose.