Slightly Yellowing Leaves (also, slugs)

I have 4 purple power plants that are about 2 and 1/2 months old.
I moved them outdoors a couple of weeks ago because they were becoming root bound etc. and I didn't really have room for them. They were grown on the windowsill, not under lights.

Recently, a few of the leaves have been yellowing slightly, it's hard to see on this photo but on some leaves it is really standing out. On some it is a bright yellow 'strip' going across the middle or the tip of each leaf in the fan. Is this nitrogen deficiency or something else?

Another problem I've been having is slugs/snails. I've put egg shells around the stem to try and deter them, but they still seem to be managing to get onto the plant. Any other organic methods that I can try?

Also just looking for some advice on how they're doing in general as this is my first grow.

Diatamecious earth is a good way to combat slugs. Also slugs are attracted to the smell of beer so putting a bowl of beer by the plants is another good way. They try to get to the beer and end up drowning.