Frieren is just going for 100% completion man. At least shes not like the immortal alien from hitchhikers guide to the galaxy who travels the universe hoping the stars will spell out a naughty word from some angle.
Magic in this word is incredibly specific though. For humans if they can imagine it they can do it. For elves if they can reason it out they can do it. There is a lot of things that Friren will be unable of creating a spell for, so she would need to find a human spell to do for her. Plus it's just the joy of collecting every spell she can. With a near endless life you need something to keep your mind occupied.
The last one she's gotten is such a weird one: spell that gives you the warm of a coat. She learned from a dude who walked around the snow shirtless, and when she tried it on Stark, he said that even when he feels quite warmth it would help him withstand the outdoors shirtless.
Frieren collecting spells has always been about the beauty of magic, and appreciating her mere mortal master's legacy. Flamme spent most of her life working on magical concepts but not exactly new spells. Frieren asked her why, and she said that it would lead to the creation of innumerable spells, so many that Frieren wouldn't be able to learn them all, even with her immortal lifespan. And so Frieren travels collecting as many as she can.
Actually changing a red apple into green apple isnt that bad. Imagine you have a garden. You can make entire harvest little later if you need to do this for some reason. Since green appless on the tree shouldnt fall that fast