Illustrator :: How To Get Transparent Pattern To Show On Opaque Background

I need to create pattern using a simple shape and a background colour – the catch is they both need to be exactly the same colour.
 
For example ideally I would like to lower the transparency of the shape to around 30% so it’s lighter. Then put down a completely opaque background colour and move the shape onto the background. Logically I understand that making the shape transparent means it won’t show up on a background that is opaque... The only other option I've found is adjusting the colour saturation however this seems to have the same affect as it being transparent. There must be an adjustment option that I don't know about?
 
Flipping the idea vice versa seems to be no problem, having a 30% transparency on the background colour and an opaque shape works well however this isn’t what I’m trying to do.
 
I know it’s possible as I’m trying to replicate a pattern background we already use. Unfortunately I only have the screen shot jpeg of the background that has been OK to use on small designs in the past. Now I’m attempting to re-create it properly as a vector so I can blow it up to 2metres x 1metre for an exhibition stand without worrying about  pixelating.