Instructions
Download the "Dirt Bloom Tweaker" that is available for free online (see Resources for link). Right click on the file after the download is complete and select "Extract All" to unzip the contents.
Double click the ".exe" file to start the program. If you get a message that the game cannot be found, click "Yes" to manually select the game directory.
Click "File" and then "Backup Files" to make copies of the game files that will be changed by this program. This is in case anything goes wrong or if you want to restore the game to its original state later on.
Click the slider underneath "Bloom Amount" at the top of the program screen and, while holding down the left mouse button, move it all the way to the left. This will set the bloom amount to zero, which means it is completely disabled.
Click the "Apply" button at the bottom of the program screen to save the changes to the game. Click "File" and then "Exit" to close the program. You can now play "Colin McRae: Dirt" with the bloom disabled.