I love #c4models by Simon Brown ! https://c4model.com/

A small change is needed to make the default color for external systems have great #accessibility . I am no expert but I have a color blind colleague that's made me aware. Default colors can be changed but lots of folks are lazy or unaware.

... and since one of the stated purposes of c4 models is to bridge across communication barriers in software engineering then I feel this attention to default colors would help further the mission!