It doesn't look like the color actually changed. It looks more like your first screenshot (the one you're calling "white") uses a bold typeface, while the second one (your "gray") uses a regular weight typeface.
Yeah, no clue what specific font it is, though it looks like the same one in both screenshots. If you are in fact using Calibri, it's likely that whatever font addon you're using is setting different game font objects (GameFontNormal, GameFontNormalSmall, etc) to use different weights/variants (normal, bold, italic, bold italic). Possibly the addon's author was experimenting with using a different font object for a few revisions; it is indeed the addon that selects the font.