Graffiti effects with blending

Yo. I’m trying to create something that looks like graffiti and so far I’ve been happy using layer blending to get the results I’m after.

Here I’ve got a solid shape over my wall which uses color dodge for the effect, and it looked pretty boombastic to me.

I wanted to add some people from a photo, and as I’m not too hot on photoshop I wanted to play it safe and use the same method.

So I played with the photo so it was made up of 2 colours (the face on the right) so i could do something similar with the layer blending. I got it looking as I’d hoped (the face on the left) but I can’t edit the colour to match my shape. Any advice would be great ether it’s how I can make the blended layer more editable, or a whole new method. I’m going crazy.

Thanks in advance!