buttons - Should all links on a website be the same color?

I'm not a web designer and would like to know about link and button colors.

About links

Should all links be the same color, knowing that sometimes the background has different color so the links are not visible enough? How do you handle this?

Is it ok to have 2 or 3 different colors for links?

About buttons

I have pretty much the same problem for buttons. Do I need to have like 5 or 6 predefined colors for buttons and deal with it on the entire website?

Right now I have a CSS file that look pretty much like this with about 10 different buttons:

login-button {
    background-color:blue;
    /***/
}
register-button {
    background-color:red;

    /***/
}
upload-button {
    background-color:green;
    /***/
}
add-instrument-button {
    background-color:orange;
    /***/
}

/*
more buttons..
*/

Would it be better to have something like this:

blue-button{}
green-button{}
orange-button{}
orange-dark-button{}

So I can reuse for eg: .blue-button for different actions. How do you guys do this?