Hi Ali, what flower are you using? It is not an instant process.
You can depending on what flowers you are using dye the petals,using cochineal or similar, I have even used a felt tip pen.

Why do you want a white flower red?
You have to use flowers with hollow stems as they take up more water.

For a second there just half reading above post I thought you had been sent a bunch of roses, just shows what sight reading does.