There is a range of support from different browsers for this issue. Typically, Internet Explorer offers fewer options for changing the default greyed-out appearance of disabled controls compared to other popular browsers. I have encountered challenges in the past trying to customize this across various browser platforms. It would be intriguing to see alternative solutions that achieve customization rather than following my recommendation not to attempt it...
I must question the motivation behind altering this feature. Users are accustomed to the standard look of disabled controls, which serves as a clear indicator that the control is inactive. I strongly advise against disrupting this familiar visual cue.
Unless there is a persuasive rationale for doing so.
One potential workaround could involve avoiding disabling the dropdown list altogether (while managing the styling). Instead, add an event handler to the change event that reverts the dropdown to its original value. You may need to store the initial value, possibly in a hidden input field, and when handling the change event, verify if the value matches the stored one. If not, reset it. Failing to perform this check could result in an endless loop as resetting to the original value triggers the change event again.