It’s now easier to experiment with colors in Mapbox Studio Classic. Click on a swatch in a CartoCSS tab and you’ll see one of these:

The picker supports all Mapbox Studio Classic’s color modes, including hex, RGB, HSL, and it even has a transparency range slider for RGBA and HSLA.

If you haven’t tried Mapbox Studio Classic out yet, download the latest version - it’s open source and publishing your map on Mapbox.com is free.