One plausible explanation for the 'yellow turning to green' corals is that the high percentage of blue/hyperviolet is stimulating the production of additional green flourescent pigments (GFPs), which allows the coral to emit the excess excitation energy as 'green'. This is a potential protection mechanism for the coral against an excess of these photosynthetically stimulating blue/hyperviolet wavelengths . When less of the blue/hyperviolet is present, the coral produces less GSP. What little GSP is produced is then blended with the coral's own whitish/pinkish tissue pigment to produce 'yellow' to our eyes (green + red = yellow). Since corals often have a blend of different fluorescent and non-fluorescent pigmentation, and not all have GFPs, this can explain why some corals turn green so easily and others retain their yellow coloration even though they are under the same lighting. Also of note is that when subjected to true 'blue' wavelengths (~470nm) or cyan (~495nm), some coral pigments emit as yellow!
The URL below is a great little chart, that you may already be familiar with, but it shows the 'excitation' spectra (from a light source) and the 'emission' spectra from the coral. Note that royal blue (~450nm) excitation can cause the coral to emit in shades of blue, cyan or green depending on the type of coral fluorescent pigmentation:
http://www.advancedaquarist.com/2012/10/aafeature_album/image014.jpg/image_full
It seems to me that in a population of different yellow Acropora kept under the same lighting conditions there may be some that stay yellow and some that easily turn green, all depending on the types of coral pigmention present. Finding that 'magic spectrum' where most, if not all, stay yellow may be quite the challenge.
Best of luck!
