I haven't really looked into this, but your guess sounds reasonable: I think Snow Leopard might have finally dropped support for 8-bit color display modes, and I'm pretty sure Fallout still used them.
I'd love to make them work again sometime, but we ported Fallout under contract and don't have the rights to publish an update even if we did have time to work on it. I hate to say it, but your best bet might be to pick up a copy for Windows (Fallout 1 & 2 are both on Steam for $9.99, or you can get
both plus Fallout Tactics for $19.99).