At the bottom of the digest email there is the “New for you” section. Those posts are linked to categories.
The categories have no background color (hard to see, white on light gray):
When looking at it through the admin panel, in “Preview Digest”, it looks like this instead:
I have inspected the template and it seems that those colors are applied via a
<span> that is
position:absolute and placed behind the text.
Gmail is stripping this style declaration and the color is never seen.
I am not sure if other mail providers have the same behavior.