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Matching a transparent color to a solid

Need a method



4:35 pm on Jan 7, 2010 (gmt 0)

5+ Year Member

I hope I can explain this right.

I have a solid blue background color on a web page. Then I have a 800px solid white centered container that is going to hold all my content.

What I want to do is have transparent blue div that while over the white, will be as close to the solid blue page background as I can get it.

Right now I am just eyeballing it. And I would like to know if there is an actual method to figuring this out. I have searched everywhere with no answers.

This is as close as I can get it.
solid blue background = #99ccff
transparent blue over solid white = Opacity 37%, #0099FF

There is no CSS alpha/trans applied. The trans blue is a transparent png image created in illustrator set to 72ppi in RGB color mode (as basic as it gets).

If anyone has a method other than just "eying it", please let me know


6:58 pm on Jan 12, 2010 (gmt 0)

WebmasterWorld Senior Member 10+ Year Member

Screengrab the image - open in image editor and 'pipette' the colour?


7:24 pm on Jan 12, 2010 (gmt 0)

5+ Year Member

Well that makes more sense... Match the solid to the merged color that the transparent over white produces!

You know what is stupid I did that for a site I did a while back. I guess I was just so dead set and concentrating so much on matching the solid (websafe) color, that I forgot it was much easier to do the reverse.


7:33 pm on Mar 18, 2010 (gmt 0)

WebmasterWorld Senior Member 10+ Year Member

Hi there CSS_Kidd,

you may find this example of interest.

There is an arithmetical method to calculate the background-color value of the div with opacity
applied, so that it matches the body background-color value.

The arithmetic needs to be applied to both the opacity div and the container div to obtain
the correct result.

The body background-color is #99ccff or rgb(153,204,255).
The container background-color is #ffffff or rgb(255,255,255).

We cannot use your opacity value of 0.37 in this case as it would produce a negative r value.
The minimum value would be 0.4.

Here is the arithmetic....

  1. body - rgb(153,204,255)
  2. container - rgb(255,255,255)
  3. opacity:0.4
  4. r value=(r*0.4)+(255*0.6)=153, which gives r=0
  5. g value=(g*0.4)+(255*0.6)=204, which gives g=127
  6. b value=(b*0.4)+(255*0.6)=255, which gives b=255
  7. opacity div - rgb(0,127,255) or #007fff

Here is the example..

<html lang="en">

<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<meta name="language" content="english">
<meta http-equiv="Content-Style-Type" content="text/css">


<style type="text/css">
body {
background-color:rgb(153,204,255); /* #99ccff */
#container {
padding:60px 0;
background-color:rgb(255,255,255); /* #ffffff */
#div1 {
background-color:rgb(0,127,255); /* #007fff */

<!--[if IE 6]>
<style type="text/css">
#div1 {


<div id="container">
<div id="div1">opacity 0.4</div>



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