Primary blue and purple are the colours that must be mixed together to form royal blue. Because royal blue is a shade of blue, all you have to do to make it is darken the blue.
Red, blue, and yellow are the three primary colours on the colour wheel. Orange, green, and purple are the secondary colours created by these main colours. The majority of colours are created by combining basic and secondary hues. The colour royal blue is a combination of primary blue and secondary purple (blue and red mixed together in equal parts).