If you pay close attention, color is a useful - but not always exactly accurate - guide to effects in Felltower.
Certain effects, and certain effect-bearing monsters, come color-coded.
The beholder fired off eye rays of various colors, including green (panic), yellow (sleep), grey (petrifaction), translucent smoke-grey (armor-bypassing damage), black (death), purple (paralysis), among others.
Traps have fired off colored rays that specific, generally consistent effects.
And some monsters are color-coded in a way that indicates their powers.
The most consistent colors are those of effects.
Purple rays, for example, are generally paralyzing. These include wands of holding, such as the one the PCs have (had?), rays from traps, and rays from the beholder, too.
Green rays have been exclusively fear or other emotion-warping effects.
And don't forget the "black fire" that so perplexes (and injures) the PCs in Felltower.
In general, if a beam/ray is of the same color as another beam/ray, it probably does the same thing.
Monsters are sometimes color-coded to indicate effects or powers.
Yellow Ravening Eyes, for example, are spellcasters.
Blue-hued monsters include a lot of winter-themed monsters, as do white ones. Red tends to indicate fire or heat.
Dragons are supposedly color-coded, but the PCs have only limited proof - a single red-hued dragon that breathed fire.
But equally, some are not - a blue beholder is merely blue. It may desire cookies more than a red beholder, but it's no more or less cold or heat themed. A red-skinned demon might be red because I though the mini would look cool red but not use fire attacks. And so on.
I find that using themes, even if just in colors, helps the players feel like there is an underlying and consistent world. And there is, such as it can be with a weird world of magic and elder beings and strangeness. But just like fire is hot and ice is cold, purple rays will paralyze you and green ones warp your feelings. And it's easier on the GM, as well.