Pedro II was blonde.
We don't know what Tomyris looked like (if she even existed), but as depicted in game she looks Eastern Iranian. On the whole in Civ6 they've had a habit of darkening leaders (Harald, Pedro, Philip, Cleopatra, Tamar), not lightening them...Also people from the Levant tend to be quite fair; Dido's clothes in Civ5 were strange but otherwise her appearance was fine.
The Ossetians are probably their descendents, so that's a good place to start. If you look at the Iranian peoples of Central Asia and the Caucasus, that's probably more or less what the Scythians looked like. And that's more or less what Tomyris looks like in-game.