Google Sheets - How can I colour a cell based on that cell value changed by user?

Hi all,

I know how to do this in VBA, but I am very new to Google Sheets. In fact this will be my first script.

No matter how much googling or surfing I do, I cannot find the exact answer I am looking for and hoping someone here could assist. Most Youtube videos I do come across seem way too advanced for me.

I have a cell C6 in "sheet 1" in "test" workbook that I would like to be highlighted in red after I have modified that cell only.
I would also like cell C6 to be a different colour and the number 150 into C6 on any change to C4. C4 is currently a dropdown menu.

So it's not quite a conditional formatting routine.

I must be searching for the wrong things / or don't know how to search properly.
Can anyone help? I am sure it's super easy for the majority of you.