There are 5 sets of numbers, each with a different color. There are 5 houses, each with a different number from a different colored set. Once you pick a number and a house, you've given the site your number. The other questions are just a distraction.

Wrong. My number was 2 and the door opened to 21. ETA: second time was a charm. Picked 6 and opened to 6

1st question narrows it down to 5 choices. 2nd doesn't matter 3rd has the 5 choices spread across the 5 houses, so you just gave it away. The rest of the questions are for fun.

Don't see how thats possible. Number 2 on the first page is red, number 21 is pink. If you picked the color red on the first page and then picked house B on the third page (which included #2) it should have registered the only number in that set that was also in the red color set. #2

This, take a screenshot before you start and you can see how it works from question 1 and on the house question.