Why do guppies have different colors in other stream's?

It depends on how much protein and vitamins are in the food they can access in that part of the river.

If a guppy has plenty of protein and vitamins it becomes very colourful and healthy. If not they can be quite dull in colour.

It also depends on what colour their mother and father were. If its father was blue and its mother was red, it will most probably grow to be either blue or red or maybe both.