Thanks for the tips, both of you. The situation was difficult because the image to be traced was a light one, and the detail was not very clear.
But what I did on the iPad Pro was to click the option to convert to paint. Though I wasn't going to lift colour from the image, just wanted to trace it, this had the effect of copying the image to be traced onto another layer at full opacity.

Nevertheless, because the faded tracing image was above it, as you pointed out in your posts, this new copied layer was still faded.

To correct this, I made the original tracing image disappear using the eye icon, and finally reduced the opacity on the "convert to paint" layer to a level which showed detail but wasn't too saturated with colour. not very elegant, perhaps, but it worked!