Q: We will finally have First Communion on Sat. morning June 27.
Are vestments white or green??
A: On that Saturday, the priest may wear white.
First Communion has no ritual book in the same way as baptism, confirmation, weddings and funerals do, for example. The Roman Missal has no Ritual Mass for First Communion, probably because the ceremony grew up in popular piety, not by Vatican authorization, and the point is that the children receive communion, which is the point of any mass.
However, the Lectionary DOES include a section among the Ritual Masses called “First Communion for Children” at #769. It refers you to readings for Christian Initiation and the votive Mass of the Holy Eucharist.
On a Saturday morning in Ordinary Time, you may use the readings and prayers from the Votive Mass #5 of the Most Holy Eucharist. The priest may wear white.
It’s different on a Sunday. A parish priest does not have authority to replace the mass texts for a Sunday in Ordinary Time with a Votive Mass, but he does have authority to replace them for a Ritual Mass such as the baptism of an infant (see Order of Baptism of Children 29 §2a, for example). If the lectionary were your only guide, you could make a case not only for white vestments, but special readings to replace those for Ordinary Time on a Sunday.
But since the missal is silent about this idea, I’d play it safe. On a Sunday, I’d use the readings of the day. I’d wear green.