I think what you could do is set up a `if/else` statement in your index.php that calls the single .php files as template parts. *Importantly you would want to put the joint condition ( blue and red) as the first in the list of conditions.* Note that the you have to:

  1.  declare a template slug to name the template part in single-red.php and single-blue.php and 
  2. use those slugs to call them with `get_template_part`.


#The code to control three possibilities

    if ( in_category('red') && in_category('blue') ) {
         get_template_part( 'single-red' ); //both
    } elseif ( in_category('red') ) {
        get_template_part( 'single-red' );  //just red
    } else {
        get_template_part( 'single-blue' ); // just blue
        
    }

or even more elegantly, if you don't care about keeping track of just reds:

#The code to control only two possibilities 

    if ( in_category('red') ) {
         get_template_part( 'single-red' ); // just red OR blue and red
    } elseif ( in_category('blue'){
        get_template_part( 'single-blue' ); //just blue
        
    }


I hope that gets your closer! Please try that and let me know if you run into any issues!