Colons should not be used as a semicolon or a comma. Most frequently, they are used in listing things. For example:
The cat had four different colored kittens: gray, black, orange and tabby.
The kittens were cute: I loved them.
This should be:
The kittens were cute; I loved them.
Colons can also be used as above where I wrote, "This should be:" If not using a comma and quotes, you can put colons in place so that the text reads more easily.