You have to root your phone. If you can install Xposed Framework on your phone, then you can install an app called Gravity Box that will let you change them. If not, then you should be able to do it manually (still need root), go to System/App/SystemUi.apk and copy to your computer, extract it with something like WinRAR then in the res/drawable (might be in drawable-hdpi or another drawable folder) you should find the icons they are PNGs, you can edit them with Photoshop or something that can open and save transparent PNG files, or I imagine you can find pre-made status bar icon packs. When you're done editing, put them back in the .apk file, put it back where it came from, cross your fingers and reboot your phone.
Honestly I thought it would be easier than that (though of course you would need to be rooted), but if it really bothers you, you can do it. There are always custom ROMs too.