For a spectacular perspective of Telaga Warna, it's worth climbing the hill above the lake. Called Ratapan Angin (and signposted as Dieng Plateau Theatre), this established viewpoint is accessible by car for around 500m, but the remaining half kilometre to the hilltop is via a steep path. At the top, enterprising locals have built a variety of selfie stations, including a rope bridge and swings, none of which mar the view, which is particularly striking around sunset.
Tours from Yogyakarta often deliver travellers to this point rather than to the lake itself so it is worth checking before booking with the agency.