It seems a rule of thumb (be it legally true or not) to treat a flashing red light (signal?) as a stop condition. MUTCD section 2a.07 https://mutcd.fhwa.dot.gov/htm/2009/part2/part2a.htm#section2A07 talks about illumination of signs. Point 08a mentions white or red LEDs on yield signs. Part 09 says "if flashed", so i assume agencies can choose either way. Has anyone ever actually seen this? I can't think of examples. This came up related to the "failure to yield" at roundabouts, but I'd be interested to see other locations too.
We've had this debate around our office for a while, especially with the roundabout situation you reference. While no one has pulled the trigger on installation, I successfully lobbied for flashing white on a Yield sign, given the nearly ubiquitous meaning of a flashing red. Likewise, I have yet to see any installed in the field. Most agencies opt to install larger Yield signs, additional signs, or add flags instead.
While it would contradict the sign color coding system, given the spread of the flashing yellow arrow and its meaning, one could also potentially argue for the use of flashing yellow.
Here's one in Goodland, Florida...though not for a roundabout:
(https://live.staticflickr.com/5585/31408836326_fdc18778be_b.jpg) (https://flic.kr/p/PRuuLN)
(I think it took me 7-8 shots to get the lights in the photo.)
They should have it flash white
iPhone
Joplin, MO does that.