I tried searching for RGB LED drivers that wouldn’t be integrated into each LEDs, hopefully meaning lower power consumption, found that one : KTD2026 3-Channel RGB LED Driver with I2C Control | Kinetic Technologies
2.7-5.5V, 24 mA 3 channels, I2c,<1uA in shutdown, 200uA with LEDs OFF, 300uA with 3 channels ON, quite better than 1mA per LED. DFN 1.5x1.5mm so it’s small enough to place on an AUX PCB, 125uA per step (192 steps) might be a bit too coarse though. Pretty cheap at 50c/pcs for 10.
The KTD2026/7 are fully programmable, constant current RGB or RGBW LED drivers with a flexible control interface. The devices are ideally powered from one-cell lithiumion/polymer, 3-cell NiCd/NiMH/Alkaline batteries, or systems with 3.3V or 5V supplies. The independent programmable constant current sinks operate without external components.
With an on-chip timing control unit, LED blink rate, fade-in and fade-out are user-adjustable resulting in unique color lighting patterns.
Ten internal registers are programmed via the I2C interface with a built-in decoder allowing individual control of the three/four LED channels’ On/Off state and current level. A total of 192 current levels are available for each channel from 0.125mA to 24mA with a 0.125mA step.