Obviously you want only a limited set of hues, but a full range of brightnesses. Check this:
It has two layers. Both layers are the same image, only colored differently with Image > Adjustments > Hue & Saturation > Colorize
The same image is pasted to the layer mask of the upper layer. It's contrast was adjusted with Image > Adjustments > Curves until a good separation was found. The mask is now this:
You can get the mask onscreen by clicking the mask icon in the Layers panel and holding the Alt key at the same time. This is needed when pasting (in place) the image to the mask.
The adjustments affect to the mask as soon as its icon is selected, no need to have it onscreen.
If the mask has low contrast or is made a little blurred, you will get plenty of mixed tones at the grey areas of the mask. Here the mask is blurred:
It can be useful to adjust the contrasts and brightnesses of the layers separately or at least add a curves adjustment layer over the lower layer.