I wondered if it was possible to create two background-colors for a div an overlay them.
I want a white background-color so that the content below this div doesn't shine through and another rgba() color painted over this white to create lighter colors per script.
Without understanding why you want this, it can be done by using solid color gradients: fiddle.
body {
background: linear-gradient(rgba(220,14,120,0.5), rgba(220,14,120,0.5)),
linear-gradient(white, white); /* first bg is on top of this */
}