Well. It took you long enough. But finally you have tracked El Circo de Desconcertante down to this door in this dismal alley.
You knew you wouldn't just be allowed in, so you've come prepared with the information you need. Now it's just a matter of saying the right thing...
Adding to the partial answer from @mike-limburg and noting that @Techidiot did almost all the work here...
The cipher:
LJLD K Y I AT O I X FJ I AW O EA K JO B P O ED K YL B LJLD K CO B W O ASN D HE
Can be deciphered by
applying the vigenere key 'TRIANGLE' because as @mike-limburg found out, the HEX codes of the colours are ASCII for 'LOOK AT THE SHAPES'
Once decoded:
SSDD X S X WA X A X SD X WD X WA X DD X W X WD X SA X SSDD X WD X D X SSA X WA
That step will be important in a bit.
Next you'll need the image from clicking on the picture of the letter with the 16 circles as well as the letter text itself. Again the shape is key here:
You'll need to extract all the "triangular number" letters from the text:
Letters: 1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 66, 78, 91 which come from 1, 1+2, 1+2+3, 1+2+3+4, 1+2+3+4+5, etc.
Those letters from the letter fill the 16 circle grid row by row:
TMIS
QXGE
DOUL
HAJN
Now you need to come back to the decoded cipher:
Starting with the X on the 4x4 grid of letters, use WASD as keyboard arrows (UP, LEFT, DOWN, RIGHT) to select letters, going back to X when the cipher says X.
The output reads:
NOT QUITE MIDNIGHT
Which is the rhyme we need to get in.
(very) Partial Answer:
By clicking on the first link, we see a picture with different colored stripes. Using a color picker, I discovered the following hex codes for each color:
076079
079075
032065
084032
084072
069032
083072
065080
069083
We can decode this numbers
From ASCII into separate letters.
From there on we read the following sentence:
LOOK AT THE SHAPES
As @TechIdiot and @Jim found out, the (triangled) shapes say:
Knock on our door sometime / To enter just say the rhyme
