I had the same exact thing happen..
not sure if heat was issue or not..
but i used 9 drops rootbeer - LC -
to 4 oz plastic..
stated with 4 drops also and looked ok but next day it was to lite.
last batch was 9 drops and seemed ok next day it was not as dark as at 1st but pretty good,,
have not made this color much but maybe watch the temp ??
never had the root beer do that with other colors ,, very odd ..