Prior to css3, we only use RGB colors. RGB colors are declared in RGB or Hexadecimal format. For Example, Red color is
RGBA colors are same same like RGB colors, but they have an extra forth value: Alpha. Alpha is opacity level of color. Thus Solid red color can also be written as
rgba(255, 0, 0, 1). But if we want to add transparency in red color, we need to use RGBA color. Alpha value is always floating number, it varies between 0 and 1, exp rgba(255,0,0,0.5)- red with 50% transparency. But unlike rgb, there is no hexadecimal notation for RGBA colors yet. For dark( solid) colors, alpha value is 1. For full transparency, alpha value is 0. And for semi transparent color, alpha value can vary. Alpha 0.25 means 75% transparency, 0.75 means 25% transparency.
<div style="background:rgba(255,0,0,1)"> color rgba(255,0,0,1)</div>
<div style="background:rgba(255,0,0,0.75)"> color rgba(255,0,0,0.75)</div>
<div style="background:rgba(255,0,0,0.5)"> color rgba(255,0,0,0.5)</div>
<div style="background:rgba(255,0,0,0.25)"> color rgba(255,0,0,0.25)</div>