Here is a much shorter, clearer statement of the puzzle I see for Reid on color perception:
1) A perceiver observing a uniformly blue sphere would only perceive something uniformly blue if one sees the sphere as a 3-dimensional object.
2) An original perceiver observing a uniformly blue sphere would not see the sphere as a 3-dimensional object
3) So, an original perceiver observing a uniformly blue sphere would see something variably colored, rather than uniformly blue.
4) No external object being perceived is variably colored.
5) So, either a) the original perception is of the sphere, but is not correct., or b) the original perception is correct, but not a perception of something external.
6) Reid’s direct realist account of perception requires that the original perceptions are correct, so, not (5a).
7) Reid’s direct realist account of perception requires that the original perceptions are of external objects, so, not (5b).
Reid is committed to (1)-(3) by the passage I quoted in the previous post.
I don’t have a source for (4), but I am not sure what externally existing object is variably colored in a perceptual situation involving a perceiver and a uniformly blue sphere.
(5) follows because the perception is either correct (and therefore not of any external object) or not. If it is incorrect, it may as well be a perception of the sphere.
(6) comes from the veridicality of perception on Reid’s picture (he goes to some lengths to argue that the sense do not deceive us), and (7) comes from the fact that Reid is insistent that the objects of perception are external objects (and their qualities).
So, this is the puzzle. Reid can avoid the puzzle when it comes to visible and linear distances, for instance, because there are two different (but related) qualities he can invoke. But there is no such distinction available when it comes to color.