No, Elphaba wasn't hard to spot at all. So Tonks was grinning when her suspicion was proved correct and it was, in fact, the other witch. A genial wave was given, though hello's waited until she was closer.
"Wotcher, Elphaba. And it's just Tonks, yeah? No miss stuff." Added with a slight wrinkle of her nose. First names (her own, rather) and proper greetings were not things that Tonks encouraged. She scooted on the picnic table, making room for Elphie in case she wanted to sit.
"You live here? And all this time I've been worrying about getting over to the library to see you!" Imagine that.