The first shot of Susan shows her blue eyes and bangs, but once we get a better view, we see that the rest of her hair is just brown.
Susan has Green hair and eyes. But then she reveals that she was wearing green contacts, and that her eyes are actually brown...
...Until she takes off her wig.
...then she pulls off her green wig to reveal that she also has brown hair.
Whenever Susan wears colored contacts, her hair changes to match, much to her bafflement.
Alice's blonde hair matches her amber eyes.
Zig Zagged: Susan, a minor character, often has long gaps between appearances, and it seems that the writers never remember her hair and eyes the same way twice. Sometimes, they're both blue, but half the time, her hair's bright green.
Averted: Susan's hair and eyes are different colors.
Enforced: Susan originated in a videogame from the 8-bit era, where color palettes and sprite complexity were limited, so her eyes had to be of the same color as her hair out of necessity.
Lampshaded: "Hair color and eye color? Wait, there's supposed to be a difference?"
Susan dyes her hair blue because she likes how it matches her eyes.
"By law, everyone must either dye their hair to match their eye color, or wear colored contact lenses that match their hair".
Defied: Susan dyes her naturally blue hair green because she thinks that it's boring to have the same hair and eye color.
Discussed: "Wow, I love how you dyed your hair blue, Susan!" "Thanks; shame I didn't dye it green like my eyes, like some sort of anime character".
Conversed: "Her eyes and her hair are the exact same shade of pink? This show sure does have some odd color choices".
Deconstructed: Susan hates the fact that her hair matches her eyes, and goes through huge measures to not let it show...
Reconstructed: ...But later, she realizes that she is fine just the way she is, so she keeps the look.