I don't know why Google changed the colors of the categories in
Google news but it looks like they actually regressed.
Each major category in Google News had their color changed yesterday. Each category is a variant of their classic Google color scheme.
The news categories now make the website look like it was produced for (or by i can't say) an 8 year old. If you don't read the actual news headlines you may thing it is a page for kids.
If memory recalls, the categories in
Google News used to have a solid background greyish color. The only colors in the category was a top border which used the Google colors.
Perhaps it's just because it's new change, but the colors just seem too distracting, keeping the user's eyes from the actual news content. If a color wheel is fired up, the colors don't even match the same scheme. The UI team really goofed on that.