I just noticed that Skritter doesn't allow hiding pinyin while studying tones ONLY. So, for example, if I want to study the tones of a set of characters without actually drawing the characters, I'm unable to hide the pinyin , because even when I check "Hide Pinyin" in the settings, the pinyin still shows up. The only time the pinyin is hidden is when I'm studying writing and tones, but it won't work for just studying tones.
I actually think it would be much better to have the option to hide the pinyin while studying tones, so that we can't cheat at all by looking at the characters pinyin and then remembering which tone it is.