If you've ever shot liquids - especially translucent liquids - before, you've likely come across the liquid getting discoloured in the neck (because there is less of it). Think of a wine bottle. Fortunately, there's an easy way to get back that beautiful colour, and it involves Photoshop.
The thing here to keep in mind is that you don't want to exactly match both colours because it will seem fake. In fact, what you want to do you is to get closer to the colour of the liquid in the rest of the body. To accomplish so, you can use both a curve and hue adjustment layer. I suggest starting with the curve adjustment tool and finishing with the hue. I will show you the steps required to match the colour of a rosé wine but they should be really much the same as for any other colours, just some other values.
So, once the curves panel is open, click on the layer mask in the layers panel, fill it with black and paint white where you want the effect to take place. This will do nothing right now but will ensure that the modifications you make later on don't affect all the image. Then, click on the curves logo and select the red channel in the drop down menu at the top. After, take the colour picker and click on the area of the picture you want to match and drag upward. This will increase the reds in the image. You can also try going to the blue channel and decreasing it in order to add more yellow. If you feel like it, try moving around the curves in the red, green and/or blue channel even more in order to get the most precise and realistic results possible.
Once you see that you're done with the curves, it's this to add a hue adjustment layer. After you have created the layer, make a clipping mask using the curves layer so that both tweaks affect exactly the same area. Head to the hue panel and try moving the main slider and see what that gives you. If it's not suitable, cancel that action by pressing Command+Z and press the Colorize checkbox and then try slider the pointer head towards the Reds. Hopefully, that helps the effect. However, if it doesn't, go to the drop down menu and select the Reds and then play around with the the sliders. Those two layers together should have helped you reach the desired colour. If you see that the edges of the layer mask are too rough, try softening up the mask using a Gaussian Blur filter. One thing you will probably need to change is the overall lightness; the liquid in the rest of the bottle doesn’t let nearly the same amount of light go through and, so, it is darker. To do so, just head to the curves layer and make sure you’re set of the RGB layer and just take a center point down a bit. You can also refine it up with a few brush strokes. If the colour is now too close to the rest of the liquid, decrease the layers' opacity until you feel it looks good. Maybe you need to change the curves values a bit; this technique requires a really good equilibrium between both adjustment layers. If all is good, you should now have a nicer looking image.
Have you needed to change a liquid's colour before? How did you do it? Let us know below!