Expression of red and green fluorescences in the same tissue
These images are not typically taken using color cameras. They are taken using grayscale cameras, using filters that allow the excitation wavelengths to hit the sample and the emission wavelengths to go to the camera.

For visualization, these black and white images are often put into the red or green channel in a digital image (TVs and computer monitors make color by mixing different amounts of red, green, and blue light: RGB). If you have red and green together that shows up yellow to a human eye, but that doesn't mean any yellow light is involved: it's just perception.

If you don't see yellow in these images it's because the red and green emission were not in the same place.