change just background color hex opacity code example

Example 1: background opacity css hex

background-color: #ff000088; <--- the 88 is the alpha
background-color: #ff0000 50%;

Example 2: background color opacity

rgba(51, 170, 51, .1)    /*  10% opaque green */ 
rgba(51, 170, 51, .4)    /*  40% opaque green */ 
rgba(51, 170, 51, .7)    /*  70% opaque green */ 
rgba(51, 170, 51,  1)    /* full opaque green */