Color powder

Color powder goes into the water in the base. It changes the water color and gives a visual effect, especially with LED lighting. Buy only food-grade or cosmetic-grade colorant, never industrial dyes. Smoke passes through the water, but the colorant stays in the water itself. It doesn't reach your lungs. Skip anything sold as a textile or industrial colorant, even if the price is tempting. Wrong product class, wrong place to economize.

Fill the base with water as usual, add a small spoon of colorant, and stir. Most colorants are heavily concentrated, so start with less than you think. You can always add more. After the session, rinse the base several times with warm water. Colorant left to dry on glass can stain, and once it sets you're scrubbing instead of just rinsing. A quick rinse straight after the session keeps the glass crystal clear.

Blue or purple colorant with a white LED gives a deep, saturated look. Red or orange with a yellow LED warms the whole setup. Green is dramatic with a dark blue LED behind it. Worth experimenting since one teaspoon is enough per session, so the cost of trying combinations is essentially nothing. Stronger color isn't better. A heavy dose just looks muddy when the light hits it.