Hi! I'm growing kale for the first time. About 2-3 days ago I noticed these weird lines on the lower leaves of my kale plants, and also noticed yellowish green splotches. What could be going on?

That looks like leaf miners.... If you take off a leaf and hold it up to the sun, you can see them chewing through the middle of the leaf.

I had them in spinach, and eventually ended up using the "rip it out" treatment. Hopefully someone has a better treatment for them.

What's your eventual plan for these?

If it was me... I'd be picking leaves now...
At my house, I pick the outside leaves and cook them leaving enough of the tender new leaves to keep the plant growing.

The leaf miners?
I wouldn't sweat them...

Those kale aren't going to be around much longer, anyway.

Thank you Sumire and Stone! My goodness Sumire, you were right about those leaf miners! I had to keep tearing away at all the beautiful leaves that had been eaten away. I haven't found an alternative solution/prevention yet.

Stone, my kale plants still seem to be growing, although not as fast as they did in spring when the temperature was cooler. My plans are simple..kale smoothies and salads