Css – Finding “equivalent” color with opacity

colorscss

Say I have a background color with a "ribbon" running over it in another solid color. Now, I want the ribbon to be partially transparent to let some details blend through, but still keep the ribbon the "same color" over the background.

Is there a way to (easily) determine, for a given opacity/alpha < 100% of the ribbon color, what RGB values it should have to be identical to its color with 100% opacity over the background?

Here's a picture. Background is rgb(72, 28, 97), ribbon rgb(45, 34, 70). I want a rgba(r, g, b, a) for the ribbon so that it appears identical to this solid color.

enter image description here

Best Answer

Color blending is just a linear interpolation per channel, right? So the math is pretty simple. If you have RGBA1 over RGB2, the effective visual result RGB3 will be:

r3 = r2 + (r1-r2)*a1
g3 = g2 + (g1-g2)*a1
b3 = b2 + (b1-b2)*a1

…where the alpha channel is from 0.0 to 1.0.

Sanity check: if the alpha is 0, is RGB3 the same as RGB2? Yes. If the alpha is 1, is RGB3 the same as RGB1? Yes.

If you locked down only the background color and final color, there are a large number of RGBA colors (infinite, in floating-point space) that could satisfy the requirements. So you have to pick either the color of the bar or the opacity level you want, and find out the value of the other.

Picking the Color Based on Alpha

If you know RGB3 (the final desired color), RGB2 (the background color), and A1 (how much opacity you want), and you are just looking for RGB1, then we can re-arrange the equations thusly:

r1 = (r3 - r2 + r2*a1)/a1
g1 = (g3 - g2 + g2*a1)/a1
b1 = (b3 - b2 + b2*a1)/a1

There are some color combinations which are theoretically possible, but impossible given the standard RGBA range. For example, if the background is pure black, the desired perceived color is pure white, and the desired alpha is 1%, then you would need:

r1 = g1 = b1 = 255/0.01 = 25500

…a super-bright white 100× brighter than any available.

Picking the Alpha Based on Colors

If you know RGB3 (the final desired color), RGB2 (the background color), and RGB1 (the color you have that you want to vary the opacity of), and you are just looking for A1, then we can re-arrange the equations thusly:

a1 = (r3-r2) / (r1-r2)
a1 = (g3-g2) / (g1-g2)
a1 = (b3-b2) / (b1-b2)

If these give different values, then you can't make it match exactly, but you can average the alphas to get as close as possible. For example, there's no opacity in the world that will let you put green over red to get blue.

Implementation

There are three different implementations: pseudo-elements, pseudo-classes, and nothing.

WebKit, Blink (Safari, Google Chrome, Opera 15+) and Microsoft Edge are using a pseudo-element: ::-webkit-input-placeholder. ^[Ref]
Mozilla Firefox 4 to 18 is using a pseudo-class: :-moz-placeholder (one colon). ^[Ref]
Mozilla Firefox 19+ is using a pseudo-element: ::-moz-placeholder, but the old selector will still work for a while. ^[Ref]
Internet Explorer 10 and 11 are using a pseudo-class: :-ms-input-placeholder. ^[Ref]
April 2017: Most modern browsers support the simple pseudo-element ::placeholder ^[Ref]

Internet Explorer 9 and lower does not support the placeholder attribute at all, while Opera 12 and lower do not support any CSS selector for placeholders.

The discussion about the best implementation is still going on. Note the pseudo-elements act like real elements in the Shadow DOM. A padding on an input will not get the same background color as the pseudo-element.

CSS selectors

User agents are required to ignore a rule with an unknown selector. See Selectors Level 3:

a group of selectors containing an invalid selector is invalid.

So we need separate rules for each browser. Otherwise the whole group would be ignored by all browsers.

::-webkit-input-placeholder { /* WebKit, Blink, Edge */
    color:    #909;
}
:-moz-placeholder { /* Mozilla Firefox 4 to 18 */
   color:    #909;
   opacity:  1;
}
::-moz-placeholder { /* Mozilla Firefox 19+ */
   color:    #909;
   opacity:  1;
}
:-ms-input-placeholder { /* Internet Explorer 10-11 */
   color:    #909;
}
::-ms-input-placeholder { /* Microsoft Edge */
   color:    #909;
}

::placeholder { /* Most modern browsers support this now. */
   color:    #909;
}

<input placeholder="Stack Snippets are awesome!">

Usage notes

Be careful to avoid bad contrasts. Firefox's placeholder appears to be defaulting with a reduced opacity, so needs to use opacity: 1 here.
Note that placeholder text is just cut off if it doesn’t fit – size your input elements in em and test them with big minimum font size settings. Don’t forget translations: some languages need more room for the same word.
Browsers with HTML support for placeholder but without CSS support for that (like Opera) should be tested too.
Some browsers use additional default CSS for some input types (email, search). These might affect the rendering in unexpected ways. Use the properties -webkit-appearance and -moz-appearance to change that. Example:

    [type="search"] {
        -moz-appearance:    textfield;
        -webkit-appearance: textfield;
        appearance: textfield;
    }

Css – Is it possible to change only the alpha of a rgba background colour on hover

This is now possible with custom properties:

.brown { --rgb: 118, 76, 41; }
.green { --rgb: 51, 91, 11; }

a { display: block; position: relative; }
div { position: absolute; bottom: 0; background-color: rgba(var(--rgb), 0.8); }
a:hover div { background-color: rgba(var(--rgb), 1); }

To understand how this works, see How do I apply opacity to a CSS color variable?

If custom properties are not an option, see the original answer below.

Unfortunately, no, you'll have to specify the red, green and blue values again for each individual class:

a { display: block; position: relative; }

.brown { position: absolute; bottom: 0; background-color: rgba(118, 76, 41, 0.8); }
a:hover .brown { background-color: rgba(118, 76, 41, 1); }

.green { position: absolute; bottom: 0; background-color: rgba(51, 91, 11, 0.8); }
a:hover .green { background-color: rgba(51, 91, 11, 1); }

You can only use the inherit keyword alone as a value for the property, and even then the use of inherit isn't appropriate here.