How can I change colors of a B&W graphic in Photoshop?

I have a black & white graphic, which I want to turn into a blue & yellow.

Black → blue,
white → yellow

I can do the first part by creating a solid color layer and change the blending options to "lighten", but then I have a blue & white image.

I have a vague idea that I should select highlights and shadows, resp., and turn the selections into masks for a solid color layer, but I don't know how to do this. Also, I'm not sure this is the best route to go.

Photoshop is version CS6.

EDIT:

I think I found it, but I'm not completely satisfied yet.

In the "select" menu I select "color range" and then "shadows". I then add a mask using the selection. I make that layer yellow. Underneath I place a solid blue layer.

The problem is that the edges are very hard and jagged, like when you convert an 8-bit grayscale to an indexed color image. So the mask seems to consist of black and white only. Any ideas how to solve this?