A fresh pair of alkaline AA cells at 3.2(-ish) V can’t light a Vf = 3 V blue LED with any authority, but I laid out an astable multivibrator circuit with a Piranha RGB LED to see how the colors looked:
Lighting all three LEDs at once doesn’t make much sense, although I did try it just for the amusement. Spoiler: red wins, even with more-or-less equal currents.
Mounting the hairball on an AA alkaline holder looks better:
Red being the only LED color making any kind of sense meant the Piranha was overqualified for the job, so I replaced all that clutter with a simple 5 mm yellow LED:
It’s shatteringly bright at 20 mA from fresh alkalines at 3.2 V and remains visible down to 1.8 V.
The original circuit schematic / layout doodle:
No surprise in any of this, as it’s why the discrete LM3909 circuitry happened, but it’s nice to have a simple LED atop some alkalines for show-n-tell. If, of course, show-n-tell events ever happen again …