Default primary colors is more an artifact of how long a package has been around. At one time using anything but pure colors was pretty much unusable for lines, when the main color output device was a pen plotter, or a low-resolution screen. The packages appear to have options for supporting any color.
Trying to use any color that there was not a pen color for used to be huge mistake. Hard to believe how much that has changed; and how glaring plots look like that adhere to that old standard; but it looks easy to change. They really should look at changing those defaults.
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