Why Is the Simpson's backyard tree blue in 'Grampa vs. Sexual Inadequacy' (S06E10)?

Why Is the Simpson's backyard tree blue in 'Grampa vs. Sexual Inadequacy' (S06E10)? - From below of washed feminine underwear hanging on rope with clothespins outdoors on sunny summer weather against blue sky and green trees

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While the show makes use of pastel colors, I don't remember this level of liberties taken with domestic plants. This tree on the Simpsons' yard looks standard in every other episode of the hand-animated run (seasons 1-13) I can remember. Could this be a coloring error?

The image is from the season six NTSC DVD.






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Why Is the Simpson's backyard tree blue in 'Grampa vs. Sexual Inadequacy' (S06E10)? - Decorative oriental lanterns of blue glass hanging on leafless tree in backyard on autumn day
Why Is the Simpson's backyard tree blue in 'Grampa vs. Sexual Inadequacy' (S06E10)? - Japanese umbrellas on floor near forest in autumn
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