Use clear, distinguishable colors for lines
Friction: Avoid ambiguity and over-design. High contrast matters.
Why this matters
Maps often become a mess of similar shades (teal vs. turquoise). For colorblind users or in poor lighting, these are indistinguishable.
Good Sense principle
Stick to primary, distinct colors first. Pattern or texture can be added for secondary differentiation, but color alone should be bold and unambiguous.