Low light training is absolutely invaluable. You won’t be able to use that light on your rifle or handgun very effectively if you haven’t had effective training.
It is not uncommon during low light training for your weapon light to become coated with a dark gray layer of unburnt powder due to the light’s proximity to the muzzle of the gun. This gradually reduces the effectiveness of your light as it becomes more and more obscured by the sooty build up.
The best way to deal with this build up is to prevent it. Put a drop of whatever gun oil that you have on the lens of the light and smear it around with your thumb until it covers the lens. When the lens starts to darken with unburnt powder, wipe it clean and reapply the oil. The oil will prevent the powder from sticking and will make it easy to wipe clean.
If you have build up that will have to be cleaned from your lens there are two methods that have been suggested to me that work very well. You can use a pencil eraser or some tooth paste on a cotton swab. Both can be used to rub the build up from the lens. I have found that the use of oil on the lens before shooting often precludes the need for any serious clean up after shooting.
Get some low light training and keep your light clean!