Full Idea
It might seem that colours would qualify both as concrete and as abstract objects. ...and geometrical points also seem to be borderline.
Gist of Idea
Colours and points seem to be both concrete and abstract
Source
Bob Hale (Abstract Objects [1987], Ch.3.II)
Book Reference
Hale,Bob: 'Abstract Objects' [Blackwell 1987], p.53
A Reaction
The theory of tropes exploits this uncertainty. Dummett (1973:ch.14) notes that we can point to colours, but also slip from an adjectival to a noun usage of colour-terms. He concludes that colours are concrete. I think I agree.