You have to modify it within the SystemUI apk. Download apkmanager (or apktools if you don't mind command line). Decompile the apk, then find the res/drawable-mdpi folder, this contains all the images used.
You may be able to use 7zip to just open the apk as an archive. Take the image you want to replace it with and give it the same name. Then drag it into the opened apk archive. Close the apk, and reboot your phone into recovery. Under mounts, mount the system, then use ADB to push the modified apk to /system/app/SystemUI.apk, but first back it up:
adb shell
su
mv /system/app/SystemUI.apk /system/app/SystemUI.apk.bak
exit
adb push PATH_TO_FOLDER_WITH_MODIFIED_APK/SystemUI.apk /system/app/SystemUI.apk
Then on your phone, unmount system, then go back to main screen and reboot.
If you get force closes, the signing was most likely borked, and signing is my weakness. So if you have issues, restore your backup (SystemUI.apk.bak), follow the above steps, but change the 3rd line to:
mv /system/app/SystemUI.apk.bak /system/app/SystemUI.apk
Note-If you just want to modify the color, drag the original icon out of the opened archive, modify the colors with whatever image program you use, then drag it back.