selecting and modifying white background in capture one

Excuse me if this is too obvious ... but I'm trying to select a neutral white background in images where I can alter the background on its own without affecting the product in the middle of the shot. I have taken a large no of images for a client that was meant to be against a straight white background ... but I used a white chopping block under the product in most shots... It's still easier to remove it in Photoshop than to re-shoot but I thought that this is something that C1 could handle quite easily ... I mean all I would like to do is select the colour range of zero chroma and then lighten it so that the edges of the block would become less obvious. The client could then select the shots needed and I would tidy it up in PS. I'm having trouble selecting it universally. I have tried selecting the product which is uniformly yellow and then inverting the mask but getting odd results and then no change at all to the background. .. .. Any clues.. Thanks!

R