This could go under a more scientific forum, but I think it's relevant to tank set-up so here it is. I was looking for something totally unrelated, and came across this article, which I will be able to access in full thru my library website next week. But I 'll copy here the abstract (and all other info for anyone wanting to get the full article) because as usual, the abstract (and the conclusion) are all you really need to read Titre du document / Document title Selectivity of color of shelter by Octopus vulgaris and O. aegina under different background colors Auteur(s) / Author(s) OKAMOTO*Masaru (1) ; ANRAKU*Kazuhiko (1) ; KAWAMURA*Gunzo (1) ; TANAKA*Yoshito (1) ; Affiliation(s) du ou des auteurs / Author(s) Affiliation(s) (1) Faculty of Fisheries, Kagoshima University, Kagoshima 890-0056, JAPON Résumé / Abstract Influence of background color in color selectivity of two octopuses Octopus vulgaris and O. aegina was studied in an experimental tank. Black, red, orange, yellow, green, blue, and white were used in a bright environment, and black, yellow and blue were used in a dark environment for the background color of the tank. Shelters of eight different colors (black, red, orange, yellow, green, blue, white, and transparent) were put in the tank and the location of the octopus relative to the shelter was observed and recorded every 30 minutes. It was found that both species chose the black, red and orange shelter more frequently regardless of background color, though there were some differences in the results of the two species. This difference was probably caused by the spectral sensitivity of the species and it was concluded that both octopuses prefer the dark color. Revue / Journal Title Nippon Suisan Gakkaishi* *ISSN*0021-5392* *CODEN*NSUGAF* Source / Source 2001,*vol.*67,*no4,*pp.*672-677*(24 ref.) Langue / Language Japonais Editeur / Publisher Nippon Suisan Gakkai, Tokyo, JAPON* (1932) (Revue) Mots-clés anglais / English Keywords Invertebrata ; Mollusca ; Cephalopoda ; Octopus vulgaris ; Stimulus preference ; Experimental study ; Behavior ; Shelter ; Color ; Selectivity ; Localisation / Location INIST-CNRS, Cote INIST : 2796,*35400009949976.0060 Nº notice refdoc (ud4) : 14151868 p.s. It makes sense to me based on a superfluous assumption- if they don't see red light very well, hence we use it to watch them at night, then that spectrum of color probably looks grey-ish to them which accommodates their desire to hide. So basically you can have a tank with reds and oranges which look appealing to us humans (compared to all black and dark) and it could still be a happy, secure feeling environment for the octo!