Hi, guy!
The general flashlight light color is white. But now there are many multi-color flashlights on the market, such as red, green, blue, and so on. When would you use a red/green/blue or multi-color flashlight? What kind of multicolor flashlight do you want?
I found that it’s fun to carry a multi-color flashlight when camping, isn’t it?
Sorry, I don't ever use a multi-color flashlight.
My flashlights are either white light or, rarely, a dedicated UV light. 
I don’t have a particular use for RGB. Just mostly for fun. Most RGB lights are lackluster. I’ve had my eye on OSTAR LE RTDUW which can push 5Amps per die 
, there just no good mcpcb or driver for it right now.
Check out this Emisar D4v2 that was modded modded by ttrev3. It has 2200k-6500k CCT, 365 UV, Laser, RGB. It’s a like a swiss army knife of the visible spectrum.
The mechanically rotated colored filters over the led is an interesting design. 
The mechanic reminds me of a different approach of Brynite T28 which physically rotates 3 different LEDs.
A headlamp or right angle flashlight with both white and red light can be useful for night vision retention and not attracting insects.
I use a AAA light with deep red emitter, on low (<1 lumens) everynite on my 3am restroom visits.
See my avatar. 
Light painting photography would be the main use case for multi-colour flashlights, even though few are aimed at this market.
when?
1. after it was really cheap to buy.
2. during some midnight astronomy.
3. Christmas.
Sometimes there’s another joy in using other colors, isn’t there?
For example, when outdoors at night, use red light to keep mosquitoes away.
Yes, bugs are phototaxis. White light is very attractive to mosquitoes and bugs at night. Red light keeps them away.
red is for night vision - but it needs to be low lumen or fairly low
blue led - may have some UV - possibly use for checking uranium glass for genuineness