If your eye is seeing the color (range of wavelengths) red when it looks at a red liquid, what color (range of wavelengths) would leaves immersed in that liquid be 'seeing'? And would photosynthesis be able to happen?

Probably brown or gray

No

Explanation:

The color of the leaf might probably be brown or gray but not green again. Color perception is a very important phenomenon in nature.

The color we see is that reflected from a body. Other part of the wavelength of the visible light are absorbed.

A red liquid is reflecting the red part of visible light and absorbing others. The green leaf subsequently absorbs other part of visible light and reflects green.

Since the leaf is dipped in red liquid, there would be a mix of the reflected wavelength and a gray/brown color is produced. Depending on the strength of the color, the red might dominate making the leaf even appear red.

Photosynthesis will be halted.

Photosynthesis is the process whereby green plants manufacture their food in the presence of sunlight using their green pigment called chlorophyll to trap energy from the sun.

Since the chlorophyll is inhibited by the pervasive brown/gray color, photosynthesis will not be able to take place.

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the answer to your question is slate