Add solid colour mask to motif/layer

Hello!

I'm being silly and have forgotten how to do something I used to do all the time.

I need to add a solid colour mask to a colour separated layer so that its really easy to change the colour to something else if needed. I feel like I used to do something simple like make the solid colour mask and then hold down shift and the down arrow and it used to merge the mask to the layer I needed to attach it to, but it doesn't seem to be the way and I can't for the life of me remember how I used to do this.

To be clear, this isnt one solid colour over the whole canvas, its masked just to one layer.

This is how it should look in the layers: 

Can anyone help me remember how to do this? (i'm using a Mac)

Thanks!