Thanks for the suggestion, but that doesn't seem to be the problem. I went through the process of colouring a low res tif and the same thing happened. I have managed to get around the immediate problem by replacing the already colored tif through Objects > Links > Change. It automatically replaced the low res file with the high res copy and took on the coloring attributes without any problems. Seems strange. It is obviously some kind of memory issue, but I can't work out what it is. The tiff file is only 3 mb when uncompressed.