You mean IE8 to IE9?
If so, you are using either Windows Vista or Windows 7, but you don't say.
Also, by sidebar I take it you mean the Calendar in Windows Gadgets?
If so, I see that mine is also orange. I don't see a way to 'fix' that.
In fact the folder where the Calendar images are located is at;
C:\Program Files\Windows Sidebar\Gadgets\Calendar.Gadget\images
And it would appear that you could edit those images to change the color, but then you may have the problem of saving your changes. as this folder may be protected by permissions.
There may be other options here;
I have a HP laptop and since I have changed EI8 to EI9 the sidebar calendar is now orange. I have used accessories and clicked on command prompt then ran as adminstrator then typed regsvr32.exe vbscript.dll and
this worked. But not now. Is there another way to fix this problem?