To make royal blue, you would need to mix primary blue and purple. Since royal blue is just a shade of blue, all you have to do to make it is make blue darker.
Red, blue, and yellow are the three main colours that make up the colour wheel. Orange, green, and purple are the three secondary colours that come from these primary colours. Mixing a primary colour with a secondary colour makes most colours. The colour royal blue is made by mixing a primary blue and a secondary purple (blue and red mixed together in equal parts).