It seems that the back of a painting cannot be classified into any kind of planks, even though it does look like planks.
Here's the screenshot comparing painting back to nine kinds of planks currently available in 1.19. Let me know if this hue is some early texture of planks, or simply a glitch.
This is art feedback. The painting back texture has already been updated in 21w11a and matches the other planks' textures (MC-160388).
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