Spectral determination of the colour and vertical structure of dark spots in Neptune’s atmosphere por Irwin, PGJ, Dobinson, J, James, A, Wong, MH, Fletcher, LN, Roman, MT, Teanby, NA, Toledo, D, Orton, GS, Pérez-Hoyos, S, Sánchez-Lavega, A, Sromovsky, L, Simon, AA, Morales-Juberías, R, Pater, ID, Cook, SL