Check out example codes for "background color with opacity". It will help you in understanding the concepts better.

Code Example 1

h1 {background-color:rgba(255,0,0,0.3);}

Code Example 2

.opacity30 {
  opacity: 0.3;
  filter: alpha(opacity=30); /* For IE8 and earlier */
}

Code Example 3

background: rgba(255, 255, 255, 0.25);

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