Use variables. They have an option where you can randomize a number between two bounds. In your case, you would use two variables. The first one would choose between 1 and 5 and then based on those numbers, you would assign what type of berry you find. Then you could reandomize another number between 5 and 10 to determine how many berries you get.
There are a few tuts on variables out there. Look them up. You won't regret it.
EDIT:
Your event would look like this. It would be action key activated with an empty second page (conditioned on Self-Switch A)
Text: Would you like to Forage?
Show Choices (yes, no)
When [yes]
Control Variables: [001: Berries 1] Random (0, 4)
Control Variables: [002: Berries 2] Random(5, 10)
Conditional Branch: Variable [001: Berries 1] = 0
Change Items: [Red Berries] + Variable [002: Berries 2]
Branch END
Conditional Branch: Variable [001: Berries 1] = 1
Change Items: [Blue Berries] + Variable [002: Berries 2]
Branch END
Conditional Branch: Variable [001: Berries 1] = 2
Change Items: [Black Berries] + Variable [002: Berries 2]
Branch END
Conditional Branch: Variable [001: Berries 1] = 3
Change Items: [Purple Berries] + Variable [002: Berries 2]
Branch END
Conditional Branch: Variable [001: Berries 1] = 4
Change Items: [Orange Berries] + Variable [002: Berries 2]
Branch END
Control Self Switch [A] ON
When [no]
Text: Come back later
Branch END