That thing on guessyournumber.com is quite easy actually and 3 of the steps are not even needed to be in there.
I'll show you how it's done
1)First of all pick 24 as your number and since 24 is black choose black also write down all the other black numbers or memorize them
2)It dosn't matter which color you choose I'm sure it just determines where the numbers are in the next step(I only did it twice so I'm not sure)
3)Look at all of the boxes and notice the all the numbers you have memorized or written down(All the black ones at the start) are in different boxes therefore if you do what it says and click on the box with 24 well the program knows you chose 24 because there are no other black numbers in that square except 24.
4)The crystals have nothing to do with anything and it doesn't matter which one you click on.
5)Whichever door you click on your number will be there because it's programmed to do that because it knows which number you chose so it just puts 24 behind that door.