Strict reading of the rule book is that this is an illegal shirt. 3-4-6 says, "The torso of the team jersey shall be the same single solid color from the base of the neck to the bottom of the team jersey."
Therefore, the jerseys of each team member are supposed to be the same single solid color.
Illegal shirts are allowed to be used at the cost of a player technical foul.
Of course, there certainly is an exception which allows a player to change a torn shirt or one which has excessive blood on it to a jersey that is similar in color to the team jersey without penalty.
However, forgetting a game jersey is not the same as having it become unusable. There is a responsibility issue involved. We should ask what life lessons are we trying to teach through HS athletics?
Would I assess the T or allow the kid to play with penalty? I guess that would depend upon how close to Christmas the game is being played.
Either way the kid is going to be able to participate.