Quote from MSDN http://msdn.microsoft.com/en-us/library/ms532930%28v=vs.85%29.aspx:
Color is expressed in #AARRGGBB format, where AA is the alpha hexadecimal value, RR is the red hexadecimal value, GG is the green hexadecimal value, and BB is the blue hexadecimal value. The alpha value controls the opacity of the object. An alpha value of 00 is transparent, while a value of FF is opaque.
This means:
#3f000000 == rgba(0, 0, 0, .25)
#7e000000 == rgba(0, 0, 0, .50)
#bd000000 == rgba(0, 0, 0, .75)
So the following CSS should produce equivalent background on IE6, IE7 and IE8:
background-color: rgba(0, 0, 0, 0.75);
filter:progid:DXImageTransform.Microsoft.gradient(startColorstr=#bd000000,endColorstr=#bd000000);
You should put the filter
property inside a conditional CSS for < IE9, otherwise IE9 seems to apply both properties and the result gets darker.
However, I'd instead suggest using a small semi-transparent PNG with the desired color as background-image
with background-repeat: repeat;
for better browser support - if needed.