How do I script a HP bar and that it shows different color when you get hit?

I want to know how to script a HP bar. I want it to work the way like others game out there like runescape do, like if I get hit 20 from a weapon and my health is 50 I want it to show yellow but if i get hit with another 20 and my health should be red which means you will need to eat/drink something to restore health. I've tried doing a HP script but had no luck because when i done the script I was stuck that the HP bar/frame HP bar wasn't changing the damage you get but on top right corner I could see it fine and going up slowly to full restore.