Service Portal does not render images or icons in colours when printing a page, it will display mostly everything in black and white / greyscale even when printing the page as PDF.

1 - Go to the portal page /sp

2 - Print the page

Notice the page will not contain any colours from headers and icons:

This behaviour is expected, by design of the Bootstrap 3.3.6 version, which is the only one currently usable in Service Portal. There are no plans on upgrading to Bootstrap 4.

The workaround consists in using a @media print {} CSS selector to overwrite the background colour:

@media print {

     .alert-danger {
        color: yellow !important;
        background-color: red !important;
    }
}