What is the diction of the short story "Scarlet Ibis"?

Diction means "word choice", so your question is rather unclear. Do you mean the significance? the style? examples of?

If you mean significance, then it is a matter of the influence the author's diction, or word choice, has on the mood of the story.  The diction is very much based on the portrayal of death imagery which creates a somber, dark, dreary mood that sets the stage for the death that takes place in the story.  His use of red images also lends to the foreshadowing in the text (specifically the connections established between the descriptions of the ibis itself).

If you mean style, then again you need to look at the strong and unpleasant word choices he makes in his descriptions, especially at the beginning when he repeatedly uses words that allude to death and cemeteries in his description of the setting.