Incandescent

A survivor's journal describes them as a cloud of minute, unflickering points of light, a bit like glowing locusts, that came out of the glare before he ever saw them. Locals in the great planar city told him the name. Their staple is the endless color of the Radiance quasiplane, yet they grow bored with it and happily devour the hues of any outsider who wanders by. Gray suits them fine, too.

One mote alone is harmless. Gathered 30 or more strong, they need only stay within about 10 feet of a victim for 1d4 rounds in a row; then the target saves vs. spell, and on a failure he and everything he carries lose their color and turn transparent.

This is not true invisibility, since the victim cannot see his own body either. He suffers -2 on all rolls involving physical action, bumping into things and misjudging every reach, jump and swing, and any spell with somatic components has a 30% chance of being botched and lost. Using a transparent item is also hard: anyone wielding one takes -2, though a transparent person with transparent gear is not penalized twice. A transparent shield gives no AC bonus, and transparent magical items stop working.

Remove curse or dust of appearance restores color, one person or item per application. Paint or dye works too, after a fashion.

Incandescents are hit only by +1 or better weapons, and as beings of energy they are immune to magic that changes physical form, such as paralysis, petrification, polymorph and disintegration. Darkness spells chase them away, and barriers like wall of force or wall of ice keep them back.

They never leave their home quasiplane, probably because they could not survive elsewhere. There they form huge permanent cloud-clusters of thousands or millions, each a fully independent collective of equals, and talk telepathically with any of their kind in sight.

No one knows if or how they breed. Rumor holds their number is fixed: they never age or sicken, so only violence can kill one, and each death permanently lowers the total. Slay them all and the race is gone.

Food is never lacking on their plane, yet many roam in hunting clouds seeking outsiders, whose colors are subtly unlike the native ones in ways only an incandescent can detect.

Deep in the quasiplane lives a rare, evil breed that eats only particular colors, and each theft changes its victim in some malicious way. The DM rolls on a table to see which color is stolen and its effect. They take one color per victim, after which that person is safe from them. Dust of appearance or remove curse restores the color and ends the effect, as does being made fully transparent by ordinary incandescents. Hue reavers are chaotic evil, very intelligent, identical to normal motes apart from their attack, and they steer clear of their common kin.