There are two sources I have found for the colors of the Consular Guard at Marengo.

First, there are descriptions of the infantry and cavalry colors in Drapeaux et Etenards by Pierre Charrie on page 75-77.

Second, there is a picture/painting of the colors of the Grenadiers a Pied in Les Soldats de la Revolution Francaise by Liliane and Fred Funcken on page 107. The painting conforms to the description in Charrie.

Lt Aune was the Grenadier color bearer at Marengo. He brought the colors out after the game half-hour fight. The flag was 176 cm square and was presented to the Grenadiers on 18 February 1800. It had a tricolor cravate with a central white diamond with red and blue triangles decorated with 15 golden eagles each. The triangles were red in the upper left and opposite with blue in the upper right and opposite. In the center of the white diamond was a golden sunburst with RF in the center in silver. Above the sunburst was a scroll with GARDE DES CONSULS in silver and below the sunburst was intertwined greenery as a garland.