My choice to use one of the two ships in a Seals Models kit of River class frigates as HMCS Ribble was decided by the decal set provided with the kit and my luck in finding a photo of a ship which wore one of the pennant numbers on that decal sheet. My decision to paint the ship model in white with Western Approaches (WA) Green and Blue panels was based on a guess looking at a black and white photo of the ship and a similar pattern I found in a book on RN camouflage.
Here's the ship kit in early stages of assembly. Most of the forward superstructure pieces are in place. The depth charge racks and minesweeping gear are also in place at the stern. I thought that putting the lattice bridge wing supports in place would be a straightforward task. Unfortunately I had to trim the parts a bit to get them into place.
I found that the aft AA platform part had gone AWOL. So I crafted a clumsy replacement and slapped it in place. 
After that I followed the TLAR ("That Looks About Right") principle forwarded by Malcom Wright in his series of books on RN and Commonwealth navy ship camouflage patterns. I went with an overall white ship with panels in WA Green and Blue. In a photo of the ship, the masts were somewhat darker than the shade of blue I got with my jar of WA Blue. I suspect in real life they were some shade of grey. 
Here's the mostly complete model. I say that since the matt clear paint I was going to apply to the decals had gone dry.
This is really a "Get it done ASAP build". I doubt the model bears any resemblance to the actual HMCS Ribble.
