Can I change a page's colors?

Yes. Show the page and click "Change colors" in the page toolbar. The editor opens with the Color tool and the book's palette (or your pencils) first; the other palettes and "Any color" (a color picker) are there too. Click a space with a color, use "No color" to make a space white, "All around" on a mandala to color every copy around the circle, "Color it for me" to recolor the whole page from the palette (press it again for other colors), or "Clear colors". Save the page: every color becomes a number, named in the key, and spaces too small for a readable number stay white. Your colors are kept when you change the book's palette. Colors from outside the book's palette are named by the nearest color name, so for exact names use the book's palette or your pencils.