Full Idea
The apple is not part of the nature of the colour, and so nothing prevents one from understanding the colour while understanding nothing of the apple.
Gist of Idea
We can just think of an apple's colour, because the apple is not part of the colour's nature
Source
Thomas Aquinas (Summa Theologicae [1265], Q85 1 Ad 1)
Book Reference
Aquinas,Thomas: 'Treatise on Human Nature (Summa 1a 75-89)', ed/tr. Pasnau,Robert [Hackett 2002], p.159
A Reaction
This helps to clarify why the procedure of 'ignoring' features is possible. It suggests that some features might be too entangled with the substance (too essential?) to be thus ignored. I can't think of an example, though. Why not?!