It will be tricky as I often don't show the reference image on the screen, but concentrate on the painting application.
If you had another device, you could work from the image if you can display it on another screen.
Also, It will be easier when colour mixing to work from a print-out.
As an alternative to buying a printer, you can save the reference image to a USB drive and print it out from a local print shop/photo kiosk.