In this exercise i filled an empty glass with water. I started with an image of two empty glasses, and and image with 4 different glasses filled with different liquids. Using the fluid that most closely matches the shape of the first glass i made a selection. copied it and pasted it on top of the image of the two empty glasses. It helps that there is a window behind both glasses in both images.
With the use of curves i adjusted the colors by removing yellow and pink out of the image of the liquid pasted on top of the glass image layer.
The finished image: