Which color formats can be used for border-color?

div {
    border-style: solid;
    border-color: red green blue orange;
}

How are these colors applied?

.transparent-border {
    border: 3px solid rgba(0, 0, 0, 0.5);
}

What does RGBA(0,0,0,0.5) create?

.btn {
    border: 2px solid gray;
}
.btn:hover {
    border-color: blue;
}

What effect does this create?

input:valid {
    border-color: green;
}
input:invalid {
    border-color: red;
}

What is this used for?