Is it possible to change the color of Upload button?

You can change the color of the Upload button by applying CSS rules:

.uploadcare--widget__button, .uploadcare--widget__button:hover {
	padding: 20px;
	background-color: #fecb05;
	border: 3px dashed #2c003a;
	border-radius: 0;
	font-family: Sans-serif;
	font-weight: 800;
}

.uploadcare--widget__button:hover {
	background-color: #000;
}

Here is an example of customization.
See it live