You can’t for Internet Explorer.
See this comment I wrote on a related topic:
There doesn’t seem to be a good way,
see:
How to change color of disabled html controls in IE8 using css
– you can set the input toreadonlyinstead, but that has other
consequences (such as withreadonly,
theinputwill be sent to the server
on submit, but withdisabled, it won’t
be): http://jsfiddle.net/wCFBw/40
Also, see: Changing font colour in Textboxes in IE which are disabled