How do you know if your N. v. viridescens is ready to transform into its aquatic form?

Hello all, new here. As the title says I was wondering if there are any signs that point to when/if a red eft is ready to shift to its aquatic form. I was also wondering if there is a way to encourage a shift to aquatic form or if there is a criteria that needs to be met for them to feel the need to do so. One of my efts color has been slowly getting darker and I'm wondering if this means he may want access to water soon, I just don't want to do it too soon because he may drown otherwise (correct me if I'm wrong here).

I'm asking here because I was unable to find the answer with a Google search.

Hi, I don't have experience with red efts but with firebellies I've just carefully tried when they have grown and been on land for a year (some raise them totally aquatic, not sure if this can be done with any species but if you want to read more you find the theme under Cynops thread) or so. First, I put water just a little bit, max. 5 cm about 1-2 inches?) and add so much plants that the little one can climb on them everywhere. I test if it will go hunting in the water (small earthworms or such live food). Slowly, during weeks raise the water level (every time only few centimeters/that is one inch?) and be sure newt is comfortable with it. Once, I was too early and finally had to take the newt back to "baby tank", it just wouldn't do anything in the water (about half a year later water was accepted).

PS. It seems also notos are being raised aquatic. Here's a link to a thread (there are others, too - go check the noto section!): https://www.caudata.org/forum/showthread.php?t=79568