Moses summoned all the elders of Israel and said to
them, “Get or purchase for your families lambs or kids
and slaughter the Passover offering.”
Exodus 12:21
Goats are usually black, while sheep are typically white. The dark goat, therefore, evokes the imagery of a life devoid of Divine light, i.e., the former life of a penitent sinner. In contrast, the white, unassertive sheep evokes the imagery of a pure life unsullied by sin and undefiled by a skewed ego, i.e., the life of a righteous person.
The fact that both sheep or goats may be used for the Passover offering reflects the fact that this offering was required of every Jew, regardless of his or her spiritual status. Whether we are righteous or on the way to becoming righteous, we all have to internalize the message of the Exodus from Egypt and be redeemed from the limitations that prevent us from progressing in our relationship with G-d.
—from Daily Wisdom 3