Have you matched your colors?
To spell it out, pick your palette carefully.
The colors of the letters in 'Knowledge' correspond to the lettered paintbrushes on the back of the card. When matched by color, 'Knowledge' becomes 'Manhattan'.
Letter by letter, you can paint a picture.
Not every color of paint is needed to find this solution...
Clue 1 - Message
Left, right, and center...
Clue 2 - Message
Left is marked to the left, and right is marked to the right.
The messages on the left and right are identical, but the subtle ink marks behind them are not. The hidden message is in the middle - the left paragraph has the letter to the left marked, the right paragraph has the letter to the right marked. In between each pair of marked letters are the ones that spell out 'Creative Thinking'
When said out loud, each word of the signature sounds like a letter of the alphabet.
The notes are signed, 'Billy Quort'
Clue 3 - Message
You'll find what you seek between left and right.
Clue 1 - Signature
It sounds less difficult than it is on paper.
Clue 2 - Signature
Sound it out, letter by letter.
Clue 3 - Signature
First name on the left, last name on the right.
Such lovely colorful stamps!
They truly paint a pretty picture.
The colors of the stamps correspond to the lettered paintbrushes, offering 'MOMA'. A quick Google search offers the information that this stands for the Museum of Modern Art, in New York. It was founded in 1929.
Color matched stamps offer four letters
A quick search may prove useful, you're looking for a date...
Cross reference with your existing knowledge.