I burnt my skin applying colgate toothpaste to my acne... Nw how to get rid of this. Can I apply lemon juice on it?

Use for Intended use. Just like you shouldn't use vagisil to brush your teeth, don't use toothpaste for zits.
Enough is enough. . There comes a time when home remedies won't work anymore. Leave lemons for lemonade and see a dermatologist.
Off label use. Off label use of toothpaste, or any other medication/compound is not recommended, lemon juice will likely lighten the wound but not effect healing. Consult with a dermatologist regarding your acne, and the "burn", and stop using toothpaste other than its intended use.