Why were the gardeners painting the white roses red?

From the text:

Two began, in a low voice, "Why, the fact is, you see, Miss, this here ought to have been a red rose-tree, and we put a white one in by mistake; and, if the Queen was to find it out, we should all have our heads cut off, you know. So you see, Miss, we're doing our best, afore she comes, to--"

At this moment, Five, who had been anxiously looking across the garden, called out, "The Queen! The Queen!" and the three gardeners instantly threw themselves flat upon their faces.