Two colors which are easily distinguishable when presented side by side will be confused when one or both must be recalled in memory. There are several reasons for this. One is that images in our mind are never as clear as an actual perception. If you try to recall your bedroom, you can get a general image of the location of large and significant objects, their shapes, colors, etc. But the image is not nearly as detailed as what you would see if you were actually viewing the room.
Color is a particular good example of memory's low resolution.
Another problem is that memory often stores perceptual information in verbal form rather than as an image. That is, a person might see a blue car. Later, the person will not be able identify the shade of blue because memory has only stored that fact that the car was blue. This process of converting images to words-like forms is automatic and people are unaware when they do it. Several authors have concluded that memory simply encodes the general gist of a scene.