304. Out of reverence for the celebration of the memorial of the Lord and for the banquet in which the Body and Blood of the Lord are offered, there should be, on an altar where this is celebrated, at least one cloth, white in color, whose shape, size, and decoration are in keeping with the altar’s structure. When, in the Dioceses of the United States of America, other cloths are used in addition to the altar cloth, then those cloths may be of other colors possessing Christian honorific or festive significance according to longstanding local usage, provided that the uppermost cloth covering the mensa (i.e., the altar cloth itself) is always white in color.
This regulation is pretty clear, and gives the leeway when you see other colors on the altar. Interesting that harmonization with liturgical coloration is not mentioned. It seems those who wrote this were less concerned with matching or complementary colors with the vested clergy.