Why does Joy glow?

Each person in Inside Out has a dominant emotion. Her father's was anger and her mother's was sadness. Riley's dominant emotion was Joy.

I think it was an aesthetic choice to reflect that Joy is Riley's most important emotion. Have you ever seen someone's face when they get really, really happy? Some people liken extreme happiness to glowing. I think they decided to reflect this in Joy.

This is discussed in an interview with the film's virtual lighting team. The shortest answer is that each of the characters are made of energy and glow slightly, but Joy glows the most because she's got the most energy:

"Around December 2013, I was telling everybody, 'All I want for Christmas is Joy's geometry light working. We deployed it with very little testing in March of 2014 into production, but thankfully, it ended up working out and we used it in every shot in the film."

“The look and design of the Emotions had to remind people that they are personifications of feelings,” says Docter. “They’re not little people. They’re Emotions. They’re made of energy — they’re made up of thousands of particles, which kind of looks like energy. We wanted to capture what emotions feel like — the shapes, the colors — as well as their personalities.”

Albert Lozano, character art director, was inspired by production designer Ralph Eggleston’s early efforts. “The way that the chalk spattered on Ralph’s pastels, it reminded me of bubbles. Joy is effervescent. ... I do a lot of collage work, so I took the image of a sparkler, added a face, legs and arms, and that felt like Joy to me, too. I knew she had to emit joy.”

Each character had a different version of the boiling effect or what Pixar called 'solidity', Anger is the most solid, vs. Joy and Fear have their particles emanating out more "because they are more 'volume' (than solid) - for example, on Fear's nose the particles come out really far, and off his nose, and on joy we went so far as to basically have her wrist transparent, and that was very intentional - it was something that Pete (Docter) wanted to see in the film, to give the idea they were made of energy particles, to look how our emotions feel - our emotions change even as we are standing still - and so the characters change even as they are standing still."

and in this interview with the film's "Lighting Lead", Angelique Reisch:

"She is the main character and we needed an elegant solution," Reisch continued. "RenderMan was working on what they call geometric area lights, or a geolight. What this light does is allow you to select a model and then turn that model into a light source. This was music to our ears. But RenderMan’s geolight wasn't scheduled to be ready for Inside Out's production in time, but after some internal 'red flags' being raised, Pixar's global technology and lighting people got together with RenderMan and were able to run it out in time.

Since Joy's the brightest character, she's the only one that casts light. For Sadness (Phyllis Smith), they relied on irradiance. But the inner glow was shared by all five emotions, with the procedural particles close to the skin made by the character department and the outer particle sim created by the effects team.

As to why her glow is blue-yellow, that's something that foreshadows her third-act revelation where Joy comes to understand that without sadness, there can be no joy. In this instance the blue glow represents Joy's Jungian "Shadow self".

I think its because of the metaphor, "Glowing with joy". But I'm not sure...