Honestly, this is a pretty minor effect, so I wouldn't make things too complicated. The "Light" spell - a cantrip - allows you to make bright light in any color for 1 hour, you can use that as a baseline. This could essentially be a variation on that; call it "Colored Lights", use the same mechanics as regular Light, and modify the effect to "You can change the color of one light source to a color of your choosing, which lasts for the duration [1 hour]."
If you really really want to make it an alchemical item, it essentially has the same rules but gets fluffed as a powder that you can add to a fire that changes the color for 1 hour. To create a powder that produces a particular color, it takes ten minutes and roll an easy (DC 10) alchemy check. On a success, you make the powder correctly. On a fail, it results in a random color. On a 1, when you throw it in the fire it explodes for 1d6 damage to anything within 10 feet (just because that's funny on a critical fail).
Cost per alchemy check - 2 silver or something cheap, for the proper mineral salts and materials.
Maybe for extra fun, have the DM make the check if you want the result to be a surprise. E.g., the cultists use green flames in their sanctum, and the PC is trying to make green torches so they can infiltrate the lair. DM makes the check, and doesn't tell players the result - "Yep, you made some powder." But when they get to the lair and use it - whoops, you thought you made green, but you actually made blue flames, your cover is blown!
I dunno, something like that.