Can we color only half of page?

Answer 1

Yeah just create a div (class="halfDiv") and add CSS:
.halfDiv{
width: 50%;
height: 100px;
background-color: green;
}
You can adjust the height

EDIT:
as Arturop mentioned, gradient would also be helpful:
body {
background: linear-gradient(90deg, green 50%, white 50%);
}

Answer 2

I think yes

By using div

Answer 3

Background color of body with gradient i'd use.

Answer 4

Yes specify using div