A landfall ability triggers whenever a land you control enters for any reason. It triggers whenever you play a land, as well as whenever a spell or ability puts a land onto the battlefield under your control.

A landfall ability doesn't trigger if a permanent already on the battlefield becomes a land.

Whenever a land you control enters, each landfall ability of the permanents you control will trigger. You can put them   on the stack in any order. The last ability you put on the stack will be the first one to resolve (As a result, you can have those abilities resolve in the order of your choosing.).

Intimidate looks at the current colors of a creature that has it. Normally, if Surrakar Marauder has intimidate, it can't be blocked except by artifact creatures and/or black creatures. If it's turned white, then it can't be blocked except by artifact creatures and/or white creatures.

If an attacking creature has intimidate, what colors it is matters only as the defending player declares blockers. Once it's blocked, changing its colors won't change that.