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-open {
    background-color: #16d061; 
.uploadcare-widget-button-open:hover {     
    background-color: #15c25a; 

Here is an example of customization: 

See the Pen Uploadcare Large Drag n Drop Area by uploadcare (@uploadcare) on CodePen.

For more information, see Visual Tweaks.