The red dotted line is the place boundary corresponding to the active knowledge panel, in this case, the knowledge panel for Julian, CA. According to Wikipedia, cited in the knowledge panel, Julian is an official California Historical Landmark.
Google Maps no longer uses "pink" to highlight a search result; instead, a red dotted line shows the boundary of the place.
As of October 16, 2023, the Google Maps user interface still varies according to user context, as it has been doing for a long time. Unfortunately, the screenshot in the question is cropped, and it's not showing the leftmost part of the screen.
The leftmost part has a vertical bar. On this bar, there is a thumbnail for each place recently searched. To the right, a panel shows the search box with the search query. Below the search box is a knowledge panel. Ref. How Google's Knowledge Graph works.
The knowledge panel
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Julian is a census-designated place in San Diego County, California. As of the 2020 census, the population was 1,768, up from 1,502 at the time of the 2010 census. Julian is an official California Historical Landmark.Wikipedia
The word Wikipedia has a link pointing to the source.
Below, there are other sections, but I will stop here as the core of the question has been answered at this point.
Related help articles
Google Maps design theme was updated after most all previous answers. This was announced in a blog article Google Maps gets a new look, Nov 15, 2017.
As of October 11, 2023, this blog article is still linked from the Google Maps Help article Use layers to find places, traffic, terrain, biking & transit.
Nowadays, the location in question, Julian, CA, doesn't have a particular fill color; the area boundary is a red dotted line.
When searching in Google Maps for a location with an area like city, zip code, district, or metropolitan area, Google Maps will navigate to the location and adjust the zoom to show the whole area. As mentioned above, the active place boundary is shown as a red dotted line. The leftmost panel will show several thumbnails if you have done several recent searches. Clicking on them navigates to the place location, showing the place boundary with a red dotted line.