I cannot change the colour of the titles of subgraphs in my panel data graph. I have created a MWE from one of Stata's example datasets:

Code:
sysuse xtline1.dta, clear
xtset person day
xtline calories
xtline calories, byopts( subtitle(, color(green)) )
Both graphs have the same title colour. Other commands of subgraph titles do not work for me as well. Is it just me?

Thanks in advance