What color does scarlet and navy make?

Scarlet is just another name for the primary colour Red.

Navy is a mixture of colours (4 parts Blue, 1/2 part Yellow, 1/2 part Black)

So if you add the two in equal parts (5 parts Red + 4 parts Blue + 1/2 part Yellow + 1/2 part Black) you will get a deep, dark (slightly muddied) purple with a decidedly red cast.