***Colors in CSS can be specified by the following methods:

Hexadecimal colors
Hexadecimal colors with transparency
RGB colors
RGBA colors
HSL colors
HSLA colors
Predefined/Cross-browser color names
With the currentcolor keyword***

Hexadecimal Colors

A hexadecimal color is specified with: #RRGGBB, where the RR (red), GG (green) and BB (blue) hexadecimal integers specify the components of the color. All values must be between 00 and FF.

For example, the #0000ff value is rendered as blue, because the blue component is set to its highest value (ff) and the others are set to 00.
Example

Define different HEX colors:

p1 {background-color: #ff0000;} / red /

p2 {background-color: #00ff00;} / green /

p3 {background-color: #0000ff;} / blue /

Hexadecimal Colors With Transparency

A hexadecimal color is specified with: #RRGGBB. To add transparency, add two additional digits between 00 and FF.
Example

Define different HEX colors with transparency:

p1a {background-color: #ff000080;} / red transparency /

p2a {background-color: #00ff0080;} / green transparency /

p3a {background-color: #0000ff80;} / blue transparency /

RGB Colors

An RGB color value is specified with the rgb() function, which has the following syntax:

rgb(red, green, blue)

Each parameter (red, green, and blue) defines the intensity of the color and can be an integer between 0 and 255 or a percentage value (from 0% to 100%).

For example, the rgb(0,0,255) value is rendered as blue, because the blue parameter is set to its highest value (255) and the others are set to 0.

Also, the following values define equal color: rgb(0,0,255) and rgb(0%,0%,100%).
Example

Define different RGB colors:

p1 {background-color: rgb(255, 0, 0);} / red /

p2 {background-color: rgb(0, 255, 0);} / green /

p3 {background-color: rgb(0, 0, 255);} / blue /

RGBA Colors

RGBA color values are an extension of RGB color values with an alpha channel - which specifies the opacity of the object.

An RGBA color is specified with the rgba() function, which has the following syntax:

rgba(red, green, blue, alpha)

The alpha parameter is a number between 0.0 (fully transparent) and 1.0 (fully opaque).
Example

Define different RGB colors with opacity:

p1 {background-color: rgba(255, 0, 0, 0.3);} / red with opacity /

p2 {background-color: rgba(0, 255, 0, 0.3);} / green with opacity /

p3 {background-color: rgba(0, 0, 255, 0.3);} / blue with opacity /

HSL Colors

HSL stands for hue, saturation, and lightness - and represents a cylindrical-coordinate representation of colors.

An HSL color value is specified with the hsl() function, which has the following syntax:

hsl(hue, saturation, lightness)

Hue is a degree on the color wheel (from 0 to 360) - 0 (or 360) is red, 120 is green, 240 is blue. Saturation is a percentage value; 0% means a shade of gray and 100% is the full color. Lightness is also a percentage; 0% is black, 100% is white.
Example

Define different HSL colors:

p1 {background-color: hsl(120, 100%, 50%);} / green /

p2 {background-color: hsl(120, 100%, 75%);} / light green /

p3 {background-color: hsl(120, 100%, 25%);} / dark green /

p4 {background-color: hsl(120, 60%, 70%);} / pastel green /

HSLA Colors

HSLA color values are an extension of HSL color values with an alpha channel - which specifies the opacity of the object.

An HSLA color value is specified with the hsla() function, which has the following syntax:

hsla(hue, saturation, lightness, alpha)

The alpha parameter is a number between 0.0 (fully transparent) and 1.0 (fully opaque).
Example

Define different HSL colors with opacity:

p1 {background-color: hsla(120, 100%, 50%, 0.3);} / green with opacity /

p2 {background-color: hsla(120, 100%, 75%, 0.3);} / light green with opacity /

p3 {background-color: hsla(120, 100%, 25%, 0.3);} / dark green with opacity /

p4 {background-color: hsla(120, 60%, 70%, 0.3);} / pastel green with opacity /

Predefined/Cross-browser Color Names

140 color names are predefined in the HTML and CSS color specification.

For example: blue, red, coral, brown, etc:
Example

Define different color names:

p1 {background-color: blue;}

p2 {background-color: red;}

p3 {background-color: coral;}

p4 {background-color: brown;}

A list of all predefined names can be found in our Color Names Reference.

The currentcolor Keyword

The currentcolor keyword refers to the value of the color property of an element.
Example

The border color of the following <div> element will be blue, because the text color of the <div> element is blue:

myDIV {

color: blue; / Blue text color /
border: 10px solid currentcolor; / Blue border color /
}