Download the scripts folder from that video and replace the paper_overlay.PNG from scripts> scripts> items> custom_texture_blend_layers with the one i provided.
then what you have to do is replace your scripts folder in steam>steamapps>user name> team fortress 2> tf with the one from the video I linked to above (SAVE YOUR ORIGINAL SCRIPTS FOLDER!). Then go in-game and use the decal tool on your objector. once it asks you to pick an image go to steam>steamapps>user name> team fortress 2> tf> scripts> scripts> items> custom_texture_blend_layers> and then choose paper_overlay. that will allow you to apply the decal without having to mess up the colors. after that exit the game and switch back to your original scripts folder. if you have any problems or need help message me.