A 168 They can't see through it, but nothing (aside from a healthy sense of self-preservation) prevents them from trying to walk through it. They'll have to survive the damage and pass the saves for the green, blue, and violet walls to actually make it through to the other side.
A 169 Yes, It's the standard save DC. However, your formula is wrong.
Save DCs are calculated according to: 10 + spell level + caster's primary ability score bonus (Int for wizards, Cha for sorcerers, Wis for cleric/druids, etc.).