A… “pocket spectroscope, which was the delight of my fellow students at H.S.H.S. [Hope Street High School, Providence]. It is unbelievably tiny — will go into a vest pocket without making much of a bulge — yet gives a neat, bright little spectrum, with clear Frauenhofer lines when directed at sunlight. Many are the times I have passed it around at school.” — Lovecraft, letter to Galpin, 29th August 1918.
He had the device for weather and possibly also his astronomy, as such a thing appears to have been specifically used in star-identification. The light of a star would split into a distinctive banding of lines, and thus the identity of an unknown observed star could be confirmed. Although possibly his was not powerful enough to split the light of a distant star. He did have a larger $15 spectroscope in his weather station though.
Note the conjunction here of: unknown | colour | space, in relation to a story like “The Colour out of Space”.