I've been playing Lascaux for years now, and I wanted to submit to the community two little rules tweaks that have considerably elevated the gameplay for me.
Have players secretly pick their color marker when they bow out of the betting round and submit their marker to the stack, instead of before the betting round begins.
This adds a little bit of adaptability and negotiation into the game. Players must still deal with the mind game of picking a color that they think no one coming after them will pick, but makes it less likely that any given player will be completely hosed by how the betting round goes. There's always something you can do if you're clever.
It also allows players a little wriggle room to negotiate with eachother. "I see it's just you and me left. Well, I see from your tableau that you want green. I want pink. Maybe we can work it out?"
Which begs the question, how much do you trust the other person not to just pick the color you want now that you've revealed your intention?
Allow players to increase the bid increment.
3 Player Ex: Joe bids 1. Sally bids 1. Ed bids 2. Joe must now put in two to stay in.
Since bidding always goes the same direction around the table, if one player has a tendency to always take a sweet pot and go out, the player to their left may never have a chance to make money, and gets his play options severely reduced.
However, allowing bid increments to be artificially increased allows players to opportunity to change table behavior, perhaps making the pot much sweeter much faster, or pricing someone out immediately. The counter balance to this of course is that whoever increases the bid is putting up more of their own capital for the next person to take.
These may sound odd, but I encourage you to give them a try with 4 or 5 players.