From Quake Wiki

Field definitions:

.float glow_color;
.float glow_size;
.float glow_trail;

Provides customizable glowing light effect on the entity, glow_color is a paletted (8bit) color in the range 0-255 (note: 0 and 254 are white), glow_size is 0 or higher (up to the engine what limit to cap it to, darkplaces imposes a 1020 limit), if glow_trail is true it will leave a trail of particles of the same color as the light.