Вариант ...
var reg = /^(rgba)\(([01]?\d\d?|2[0-4]\d|25[0-5])\,([01]?\d\d?|2[0-4]\d|25[0-5])\,([01]?\d\d?|2[0-4]\d|25[0-5])\,([1]|[0].[0-9]+)\)$/;
var str = 'rgba(255,255,255,0.9)';
var arr = str.match(reg);
if (arr) str = arr[1]+',{r:'+arr[2]+',g:'+arr[3]+',b:'+arr[4]+',a:'+arr[5]+'}';
alert(str)