Заполните фоновый цвет слева направо CSS
div {
font: 22px Arial;
display: inline-block;
padding: 1em 2em;
text-align: center;
color: white;
background: red; /* default color */
/* "to left" / "to right" - affects initial color */
background: linear-gradient(to left, salmon 50%, lightblue 50%) right;
background-size: 200%;
transition: .5s ease-out;
}
div:hover {
background-position: left;
}
Tired Tarantula