Change back colour of a cell when its content matches content in another sheet

As the user types text into a cell I want a search to be made on another sheet (or even on a different part of the same sheet if this is more efficient). If matching text is found I want the first cell's back colour to change.

I'm new to Excel VBA and macros but have done some VB programming in the past.

What is the best way to approach doing this, please?

I agree with @KOKOESK that Conditional Formatting is the way to solve this.

However, the cell will not change colour as you type - only after the user has hit the return key.