I think that is ok, maybe, in general. But as I understand it, phenol red is only good for testing between about 6.0 to 8.0
In soil you want to keep things at about 6.5, and in hydro at 5.8
In soil or in hydro, things can easily get well below 5.8 and well above 8.0 and so it seems it might be able to give a decent idea of what is going on in soil as long as things are staying around optimal, for soil, say keeping things between 6.0 and 7.0. But it would be lacking in tracking things close around the ideal range for hydro, say trying to keep things between 5.4 and 6.2.
There is a bottle of green drops that works very similarly,isn’t really any more expensive than phenol red kits, and would give you a better reading pH spectrum closer to what you want for your plants.