Does protection from a color prevent damage from a creature of that color with trample?

Protection from a color does four things, remembered by the acronym DEBT: the protected permanent or player can't be Damaged, Enchanted/Equipped, Blocked, or Targeted by sources of that color (CR 702.16b). The 'D' means all damage from that color source is prevented.

When a creature with trample is blocked, the attacking player must assign lethal damage to each blocking creature before assigning any damage to the defending player (CR 702.19b). 'Lethal damage' is determined by the blocker's toughness minus any damage already marked on it (CR 702.19c).

Here is the key interaction: even though protection will prevent the damage assigned to the blocking creature, the game still requires the attacker to assign at least enough damage to the blocker to be lethal before prevention applies (CR 510.1c). Prevention effects are applied after assignment. So the attacker cannot simply assign all damage to the protected player by claiming the blocker will prevent it anyway.

Example: You control a 1/1 creature with protection from red. Your opponent attacks with a 5/5 red Trampler. You block. Your opponent must assign at least 1 damage (lethal for a 1/1) to your blocker and may assign 4 to you. When damage resolves, the 1 damage to your blocker is prevented by protection, and the 4 trampled damage to you is also prevented by protection. Your opponent's trampler effectively deals 0 damage in this scenario.

In summary: protection does not stop the assignment rules for trample — the attacker still assigns lethal damage to the blocker first — but once damage resolves, all of it from that color source is prevented, protecting both the blocking creature and the player/planeswalker behind it.