Celebration?

Since the cup was not white like regular cups, he did not notice it at once, but when he looked at the corners of the cup, their lavender color was darker where the tea was touching and its upper unfilled part was a shade lighter.

Many could confuse it with the design, so it took him some time to recognize it. 

He poured the tea in the other cup and touched the traces of the cup with cotton rolled on a stick in his hands.

He then dipped the cotton in another liquid where it changed the color from lavender to black instantly.

"The cup was dipped in the poison for a whole day so that it could absorb a part of it. Then it was used to serve tea to her majesty.