[accessibility] The colours of start, via, and end icons are not colour blind friendly

[boothym]

Crossposting from: openstreetmap/openstreetmap-website#2787

Currently the only difference between the start and finish directions icons is the green and red colours.

This is obviously not good for colourblind users - there should be something other colour to differentiate between them.

For example, cycle.travel uses a chequered flag for the finish icon:

[DennisOSRM]

I experimented with these colours to be fully color blind friendly.

[boothym]

Blue/purple can be a problem.

The OSM website still uses green and red but now the markers have an arrow to start and flag to finish.

[DennisOSRM]

Screenshot for reference