Let's talk about bioluminescence or similar as an cosmetic enhancement.

[ghostunderasheet]

Plus'
Badassery or sex appeal.

Negatives
It would be hard/inconvenient to hide in the dark.

Would it be a tattoo or genetic mod or a cybernetic enhancment?
What would something like that cost?
How would you use it?
Would you as a gm allow it?
Would you as a player character want such a thing?

I don't think such a thing would be bright a light source.

[Alledisil]

Depending on the flavor of how you got this ability, either make it dependent upon their emotional state (arousedness or panic), physical state (adrenaline or movement) or mentally controllable. Make it a skin augmentation and you can't install dermal plating (trade raw power for cool flavor)

As GM, I'd give it 5ft dim light as a move action to douse or ignite if its controllable. The Illuminating weapon fusion gives your weapon the bright property (dim light shed for 20ft for 6 seconds) for 120c, so for shedding 5ft dim light constantly i'd say around 100c.

Maybe an android could have their circuit 'tattoos' enhanced to glow more brightly.

Solarian's solar mote basically functions like this, and it's not OP.

[Steven "Troll" O'Neal]

Armory says 75 credits for a tattoo. I'd go with that. They already have florescent tattoos, why not luminescent ones? As for not being able to hide, that's not true. Most armor would render any dermal illumination mute.