Albert Barnes Commentary Numbers 19:2

"This is the statute of the law which Jehovah hath commanded, saying, Speak unto the children of Israel, that they bring thee a red heifer without spot, wherein is no blemish, [and] upon which never came yoke." — Numbers 19:2 (ASV)

A red heifer – Red, to foreshadow humanity's earthly body, just as the name Adam alludes to the red earth from which the human body was formed.

Without spot, wherein is no blemish – As with sin offerings generally (Leviticus 4:3).

Upon which never came yoke – So here and elsewhere (see the marginal references), in the case of female victims.