" Is there any problem with using 'color'?"
In
xline((1/(freq*24*60*60))*1e9,'color',[125,0,251]/255 , [num2str(freq), ' days'])
yes, there is. The named parameters for xline must follow the last of the input parameters and the second, must be a linespec character or string variable if the third for the label is used. It isn't flexible-enough in parsing inputs to mix up the order. To use a color triplet and the name as an argument, you need to write the above as
xline((1/(freq*24*60*60))*1e9,'-',[num2str(freq),' days'],'color',[125,0,251]/255)
where the linespec is the linestyle '-' character.
One slight difference that makes the name a little simpler could be to use the new(ish) string class that overloads the plus operator--
xline((1/(freq*24*60*60))*1e9,'-',freq+" days",'color',[125,0,251]/255)
NOTA BENE: the use of the " double quotes around the string constant.
