Starting sometime around the 3rd century, the Roman Legion ceased being purely "Roman" in that the soldiers were primarily recruited from foreign tribes. The names of the legions from this time bear out this fact. However, a tribal name like "Cornuti," for instance, did not mean that only Carnutes were in that legion. Men from different tribes found themselves mixed together.
Due to logistical reasons, the soldiers were not issued standard uniforms. The men were given an allowance to buy their own clothing and armor. To give uniformity, the troops of a particular legion probably all had the same color helmet crests and shields. Shield blazons (graphic designs) were recorded in a document used by Roman administrtors called the Notitia Dignitatum. An original of this scroll does not exist. It was, however, copied by monks in medeival times. As a result the colors may not be exact.
The top shield corresponds to the Celtae Senio, a senior infantry unit from the Western Empire. The bottom shield corresponds to the Batani.
Phil Barker published an excellent handbook for wargamers. The Armies and Enemies of Imperial Rome features all the shields from the Notitia Dignatum plus other Late Roman shields. The line drawings are in black with a key for color.