How do I remove the background and change the colour of an object (science figure)?

Not a specific answer to this question but since you are a scientific user, you may want to check out the free ImageJ/Fiji (originally written at the NIH.) It is a science-oriented image editor that goes way beyond Photoshop in many features.

https://imagej.net/ij/

https://fiji.sc/

To select neuronal spike responses, open the Channels panel and Ctrl + click on the Red channel, for example. This is typically the channel with the highest contrast. For now, this is all you need to do and know.

Once you have made your selection, return to the Layers panel. Ensure that the layer containing the spike is selected, then click on 'Add layer mask' at the bottom of the Layers panel.

The third suggested step is to create a new Fill layer above the now isolated spike and clip it to the layer containing the spike. Choose a color of your preference. This method is recommended because it allows for easy color changes on the color fill layer.