Easy tutorial to change background color onclick or hover method

it works with the background color change but if i try to make <button id=btn9> and then put btn9 instead of background, it dont work what am i forgeting? im good at coding but it has been a while - help please. need a button that changes from black to red for a mobile app that i have made. thank you in advance. pls respond.

it doesn't work.....

can u make a button change color when its clicked and when u stop holding left click the button changes back to its original state?

awesome video
please make and upload more video