My apologies for the delay in posting a reply. Short version: it couldn't be helped ) :
So, about frost in Boulder on the night of Dec. 25 -- Actually, conditions did allow for frost if the low was 32* F since frost forms when the dew point is at or below freezing. Temperature is normally measured above the ground, where the air is warmer, so ground temperature that night was below freezing. As well, some microclimate areas would have had air temperatures below 32* F . (I offered a low of -12* F based on posts in an old WS thread by someone who lived in Boulder at the time. I just read the darn thing, but I can't locate it now to share, so I'll go with your 32*.)
Below are the requested links to articles stating that frost forms at night. They're listed just as I found them in the search results; i.e., they weren't cherry-picked. In all of them the
formation of frost is explained solely in terms of the conditions under which it occurs, not in terms of specific times when these conditions are met. None of them mention frost forming shortly before dawn. If it were true that frost only, or usually, forms at or near that time, it is more likely than not that that fact would have appeared in at least one article.
http://www.newton.dep.anl.gov/askasci/wea00/wea00039.htm
http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Frost.html
http://www.weatheronline.co.uk/reports/wxfacts/Hoar-Frost.htm
http://archive.lib.msu.edu/tic/mitgc/article/2000151.pdf
http://www.metoffice.gov.uk/learning/frost/forecasting-frost
http://www.gardening.cornell.edu/weather/frost.pdf
http://education.nationalgeographic.com/education/encyclopedia/frost/?ar_a=1
http://ag.arizona.edu/pubs/garden/az1002.pdf
http://www.ces.ncsu.edu/hil/hil-705.html
http://www.awis.com/Misc/Fact_Sheets.htm
http://www.atmos.washington.edu/~cliff/Roadway2.htm
The evolution of your comments about the frost is curious. Earlier on, you say the intruder would have come and gone before the frost formed; then, that there couldn't have been frost; and finally, that the frost, which seems to have been there after all, doesn't tell us whether or not someone was on the balcony. I'll try to answer you at the different stages.
Is it possible that there was someone on JBR's balcony the night of Dec. 25 - 26 who did not leave prints in the frost observed there by police on the morning of the 26th? Yes, a case can be made for that. However, I don't think the case can be made successfully based on the assumption that frost did not form until well after the time of the murder, or the assumption that frost could not have formed on the balcony that night at all.
I agree that the frost in itself doesn't tell us anything for sure. I'm guessing we also agree that one detail considered apart from the context of other evidence rarely does.