An 'unsigned char' is a 'byte'.
Since 'brightnessRed' will not be larger than 255, this could also be 'byte'
Keeping a variable's 'type' small helps save RAM, however, I suppose in this small project RAM usage will not be a problem.
It’s just good practise.
'byte' and 'char' one byte
'int' two bytes.
'long' four bytes
BTW
pinMode(pin_A1, INPUT); <———<<<< at power up, inputs are already set to INPUT