hello.do not worry about the colour of your stool.change in colour may be some times related to the type and colour of food but most of the times it is more or less same no matter what food you have taken.
discolouration with diarrhoea
is a sigh of gastrointestinal infection
which we call gastroenteritis
(in your case-enteritis as you do not have vomiting
) which you might have acquired from the burger and pizza which may have been cooked under unhygienic conditions or by partly cooked nonvegetarian contents if any.
so watch for the stool and stool frequency, drink adequate boiled and cooled water, avoid outside food, eat bland nonspicy diet and get examined by the physician if the frequency and amount of stool increases (say 5-7 times per day)