The goal colors actually have nothing to do with how much time is remaining (days of safety buffer).
The colors are determined by the quotient of your current headroom (measured in units not days) and your current road slope. (For weight loss goals there used to be (still is?) automatic adjustments to the road width based on data you enter) This determines your “lane” on the “road”. The positions are colored red (off the road bad), orange (wrong lane), blue (right lane), and green (off the road good).
TL;DR for a goal whose slope never changes (implying also that you never derail) the colors correspond to number of safe days. But if you ever change the slope (or you derail), all bets are off and you will need to do some maths to figure out what the colors mean.