Look at Google Maps, Bing Maps, HERE Maps (which is partly connected to Bing), and Apple Maps.
These are the market leaders. This is what users see. This is what their expectations are based on.
You go to each website, type in a route, and see what are the icons.
Quick research (10 minutes)
- HERE: lilac for start, "finish flag" pattern for finish
- Bing: green for start, red for finish (with A and B used)
- Google: circle with single stroke for start, circle with a stroke and a dot inside for finish (actually, this is UML convention, the finish looks like a target)
- Apple: green for start, red for finish
- iGo: ?? for start, "finish flag" for finish
- MapQuest - green for start, red for finish (with A and B used)
- Yahoo! - green for start, red-orange-ish for finish (with A and B used)
- Yandex - red for start, blue for finish (with A and B used)
- Mail.ru - a play sign for start, "finish flag" for finish (both blue)
- Baidu - green for start, red for finish (likely words start and finish written in)
- Mapy.cz - green for start, red for finish
So, three conventions are profound:
- green for start, red for finish
- various things for start, finish pattern for finish
- A for start, B for finish
Google Maps seems to be a deviation, but they are de facto market leaders, I guess, if they used a dog for start and a cat for finish, people would get used to that too.
I suggest to pick some which supports your thinking, do some screenshots, and present it as a comparative market research.