Looking for the name of the sci-fi book/short story that started my love of Sci-fi

So for years I have been trying to search for the book(s) that captured my imagination with sci-fi.

I found the book in my middle school library some time between 1979-1983. The book may have been from the 70's, I doubt earlier. It was a short story science fiction series geared toward grade school, elementary school kids. The stories seemed grown-up to me because they didn't always seem to end happily.

Stretching back my mind, the one story that captured my imagination the most was of this boy traveling to another planet where grass, rivers and the sky were different colors like purple, green and red. Another story may have involved kids with weird space pets.

I can describe the books, from memory, as being thin hard-back books that were tan in color and had a little area on the front of the book with an illustration of a moment from the story within. I remember there being at least 3 or 4 books that my library had, with more in the series.

I don't remember if they were by the same author, but it seemed to be a series.

I know this is a shot in the dark, but I even went back to my elementary school and couldn't find the book.

Would appreciate anyone with any ideas . . .