Hi there,
for the standard color overlay you have to make sure that the color you have selected has a lower opacity ( transparency ) set in the header element.
To add a gradient overlay would require custom CSS like this:
.page-hero,
.inside-page-hero {
position: relative;
}
.page-hero:before {
content: '';
position: absolute;
top: 0;
left: 0;
width: 100%;
height: 100%;
background: linear-gradient(0deg, rgba(255, 255, 255, 0.5) 50%, rgba(100, 167, 180, 0.5) 100%);
}