Blending mode questions

Q1: Whilst using the blending modes how do you know which colour to use as the blend?

Q2: Whilst applying a blending mode how do you create it that it applies just for that one fish and not over all the other fish?

Some is experimentation, but this is a great resource for comparison (scroll down the document to see comparison images) and further understanding of how blending modes work; please see https://helpx.adobe.com/photoshop/using/blending-modes.html. For Computer Graphics, Photoshop Homework 3, which I assume you are referring to, each layer can have a separate blending mode, so as long as each fish is on its own layer, you can apply separate blending modes. Let us know if that doesn’t help!

Thank you so much. I managed to figure it out i thought you have to have an additional layer, apply a colour and a blending mode for each one. I realised that you just apply a blending mode straight to the layer with the picture which doesn’t affect the other layers. Thank you so much for your reply it was very helpful.

So glad to hear - you’re welcome!