As a non colorblind person, I would like to understand how this image could have been modified to include our colorblind brethren.

In general it is a good idea to use colour gradients that monotonically increase (or decrease) in brightness in addition to (or instead of) hue (see here for an in-depth comparison of different colour maps. It's from a Python package, but it shows some interesting plots comparing different colour maps when it comes to brightness vs. hue). This isn't just useful for colour blind people, but also helpful when printing in black-and-white.

If you absolutely have to use a diverging colour map, you might reach most people by using blue as a major component of one, but only one of the two branches (the map in the OP uses blue as a major component of both branches, which is why red/green colour blind people can have a problem with it). That way most colour blind people should be able to distinguish the branches, since blue colour blindness (Tritanopia/Tritanomaly) is much rarer than red (Protanopia/Protanomaly) or green (Deuteranopia/Deuteranomaly) colour blindness.

Apart from that it is also possible to mark information visually in other ways than by colour, e.g. by shapes and patterns, like dotted or dashed lines for line graphs, shaded or dotted areas for bar and area graphs, or different geometric shapes like crosses, diamonds, and circles when plotting individual data points, but that is probably more useful when different sets of data are plotted in the same graph.

Rules:

Rules: