An examination of the original Greek apparently concludes that those at Colossae were keeping the Sabbaths and Feasts as a "shadow of things to come".......Jesus had already come and gone, so to speak, but they were still celebrated as a "shadow of the good things to come" because the autumn feats and what they represented had yet to be fulfilled.
Can you expound on "an examination of the original Greek"? What is it you believe looking at the Greek tells us that context in our translations does not?
Right. Christ was sacrificed once. His sacrifice continues to cleanse us as we "keep the feast" (purge sin from our lives) in him each and everyday; not just during the Passover week. This is what Paul is saying in 1 Corinthians 5.
Then there is his reminder to the Colossians:
Therefore do not let anyone judge you with respect to food or drink, or in the matter of a feast, new moon, or Sabbath days—these are only the shadow of the things to come, but the reality is Christ!