Right click and select Color Picker, or hit CMD-SHIFT-C/CTRL-ALT-C to open it. Currently reads HEX, HEXa, RGB, RGBa, HSL, HSLa, HSV, HSVa, VEC3 and VEC4 colors – and is able to convert between the formats.

It also inspects Sass and LESS color variables. Just open the Color Picker with the cursor at a variable and it'll look up the definition for you. From there, you can click the definition and go directly to where it's defined.

Open Atom Settings, go to Packages in the left hand sidebar, and press Settings on color-picker to open the list of settings available for the Color Picker.

• Abbreviate Color Values: If possible, abbreviate color values, like for example “0.3” to “.3”, “#ffffff” to “#fff” and “rgb(0, 0, 0)” to “rgb(0,0,0)”.
• Automatically Replace Color: Replace selected color automatically on change. Works well with as-you-type CSS reloaders.
• Preferred Color Format: On open, the Color Picker will show a color in this format.
• Serve a random color on open: If the Color Picker doesn't get an input color, it serves a completely random color.
• Trigger key: Decide what trigger key should open the Color Picker. CMD-SHIFT-{TRIGGER_KEY} and CTRL-ALT-{TRIGGER_KEY}. Requires a restart.
• Uppercase Color Values: If sensible, uppercase the color value. For example, “#aaa” becomes “#AAA”.

• Selectable list of the current project color variables