There seems to be a machloket between Rashi and Rambam concerning the final color of tekhelet. Rashi (in the name of R. Moshe Hadarshan): midnight (dark) blue; Rambam: midday (light) blue. Is it possible to dye with the Murex to obtain both these shades, or only one?
You are correct that there is a machloket as to whether the color should be dark or light. Rav Yehuda Rock writes about this in his article in Techumin and his latest book called “Eved HaMelech” – you can see the relevant section in his article on our website (https://www.tekhelet.com/rak.pdf) see the bottom of p.17 “Behirut Hatekhelet”. To cut to the chase – he states that the issue is “not maakev” – that is, one still fulfills the mitzvah whether it is light or dark. And yes it is possible to achieve all the shades from very light to very dark using the Murex trunculus – it is simply a matter of more or less dilution of the dye solution. – Mois Navon.