Adding to the partial answer from @mike-limburg and noting that @Techidiot did almost all the work here...
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'
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:
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.