// set up pointer: a "variable variable"
int *colour;

// set the r/g/b vars to index a pin
int red = 1;
int green = 2;
int blue = 3;

//given x some decision:

switch(x){
  case something:
    colour = &red;
    break;
  case something:
    colour = &green;
    break;
  case something:
    colour = &blue;
    break;
}

// write some value to the variable variable. yucky php.
analogWrite(*colour, value);