Placing coloured Grayscale images on coloured backgrounds

I noticed something recently, a couple of Indesign iterations ago. Previously, I would place a grayscale tif or jpg on a flat colour and set colours to match - the foreground colour being the colour of the image, the background colour set to match the rest of the page colour. All good for years.

The thing I noticed recently was this process was no longer reliable. Edges were appearing in the exported PDFs. They weren't teh frame edges necessarily, but the picture edges, if it didn't run right to the frame edge. Checking with Print Production tools, sure enough the page background and the background colour in the placed images were not the same. I tested with fully white images. Same result. The white area in the grayscale image, when recoloured using swatches in Indesign, was coming out lighter than the surrounding areas.

Why post this? Because I stumbled on a solution that may help others. I set the background of the frame holding the image as a tint 1% stronger than that of the surrounding areas of the page. In other words the page is 35% green, then the box my image is in is set to 36%. 

Tested in the PDF again, Print Production tools. The output separations match exactly. It's 1% difference so very hard to see in the screengrabs (the first has the frame, second it's gone), but it does print unless corrected.

It's a stupid thing, but there you go.