@jane-n This bug started a few releases before Vivaldi 6.6 was released
The steps to make the bug happen are random (I think), and these are the three ways I've found to reproduce the bug.
Click on the search bar and type in a word, then click on a link that is saved in the bookmarks.
Click on the search bar and type any word And accept.
Open a new tab and on the Speed Dial , click on a web page.
The first option is more common for the web page to be displayed in black/white. And the third option is a bit frequent.
When the screen is black/white, it doesn't matter to reload the web page, it will remain the same. In order for the web page to be displayed correctly, all you have to do is change the tab and it will be displayed.
I've been using Vivaldi Snapshot 3335.22, It also has the white/black screen issue, but it's much less prevalent than previous versions including the stable version.
Sorry if I have bad spelling, I'm using a translator to translate the text from Spanish to English.