ERDAS Imagine - missing colour column on greyscale image

I’m having trouble with a greyscale NDVI image that I’ve imported to EI. I want to set the colours manually but there is no colour column to work with.
When I import the file it just loads directly, no loading window opens and there is no option to load as pseudocolour.
The image was generated from a satellite image via a model that was supplied to me. I’ve used stretch values of both 2 and 8 (as recommended by my tutor) and it makes no difference.
A note I have from doing this previously says "go to the raster group/table tab/column properties and check the box marked editable next to histogram" but that just makes EI lock up.

Any suggestions?