The returns to the presented colors show that it was hardest to get back to the presented yellow, easiest to get back to the presented blue, the averages (216 exp. each), being: 

Sec. 
To yellow,   1.67
" red,      1.61
" green,    1.58
" blue,     1.47

The facts as to blue and yellow shown by these four tables of averages may be expressed also in this way: 

If a blue square was shown, it was easier to change the blue memory image into the other colors, and also easier to get back the blue memory image after such changes, than if any other of the three colors was presented.

If another color than blue was shown it was harder to change the memory image of that color to blue than to any of the other colors, and also harder to get back to the memory image of that color from blue than from any of the other three colors.

If a yellow square was shown, it was harder to change the yellow memory image into the other colors, and also harder to get back the yellow memory image after such changes than if any other of the three colors was presented.

If another color than yellow was shown, it was easier to change the memory image of that color to yellow than to any of the three other colors, and also easier to get back to the memory image of that color from the yellow than from any of the other three colors except red.

If we combine all the changes into a color (both changes from another presented color and returns to this color previously presented) we find that changes to green are hardest, to yellow easiest.  The averages (for 432 exp. each) are,

Sec. 
To green,     2.03
" blue,      1.99
" red,       1.97
" yellow,    1.92

The changes away from a color (both from this color previously presented and from this color to the other previously presented colors) show that it was hardest to get away from yellow, easiest to get away from blue, the averages (for 432 exp. each) being: 

Sec. 
From yellow,   2.06
"  red,      2.02
"  green,    1.94
"  blue,     1.88

As for the subjects, all six found yellow the easiest to change into, one finding red equally easy.

SUBJECTIVE.

For seven of the subjects, mental repetition of the name of the color (usually accompanied by articulatory movements) tended to bring up the color, and one other subject occasionally used this method of bringing about a change that was difficult.  With D. the color did not come at repetition of the name.  G. was assisted by auditory recall of the name.  Nine subjects reported a feeling of strain, usually in the eyes as of focusing, occurring especially when there seemed a difficulty in producing the desired change.  The tension attended almost exclusively changes of the presented color, not restorations of that color.  For D. this strain was considerable, for G. there was also an after-feeling of strain in the head.  For G. the image was clearest when the feeling of strain was least, and J. secured the promptest and clearest results when he could most nearly rid himself of anxiety as to the result.  K. in one instance (a change from green to yellow) became conscious of the setting of his jaws and motions of feet and body in aid of his attempt.  H. frequently had the feeling of physical fatigue.