Related Products are not related?!

Hi Guys

I wonder if anyone can help me. I have listed ink cartridges on my site and of course typically 1 colour is normally related to three other products (colours). Therefore, When I list a certain cartridge I am adding the other 3 x related products in the woocommerce product listing tabs. However, when i am on the cartridge page the related products are pretty random?!

Any help will be gratefully received.

Jim