Last instar caterpillars of the large marble and the olympia marble are quite similar.  With the trained eye, you can see the sublte difference in the size and hue of the gray lateral stripe above the white lateral stripe.  In E. olympia, this gray region is larger whereas the color of the yellow stripe above that stripe is much duller as compared to E. ausonides coloradensis.  Nevertheless, when these two larvae approach their time to seek pupation and change color, the large marble turns tannish; whereas the olympia marble gets much darker.  The differences then between the two marbles become obvious.