Is there an "official" standard usage of Red and Green on differential gene expression heatmaps?
As others already pointed out, there is no official heatmap color scheme. I am not sure there is an official color scheme for any other types of plots either.
However, heatmaps seem to be far more standardized. Traditionally, heatmaps have been red-black-green as you originally stated. Most heatmaps from the microarray era and early RNA-seq are like that. You can have a heatmap for any type of data, but the majority is for gene expression. A lot of tools and tutorials were written back then, so they have that color scheme. From about 2008 to 2012, most people switched to the red-white-blue (or red-yellow-blue) color scheme. Switching green to blue avoids the color-blindness issue. Switching black to white is better for the ink if you print your papers and generally looks better on white background.
I originally found red as high to be confusing because I was thinking in financial terms (if the market is red, it's down). Now I think of it as a water faucet (red is hot, blue is cold).
Edit: In the last year or so, the viridis palette started to become widely used. It's far from the standard, but could become more popular in the future.