Why do colors look weird?
Actually, that's the color of the green patina (rust if you want) formed when bronze (in this case) is weathered/exposed to air over a period of time.
And I checked that colors available for the cannon barrel parts, like 3063 comes in both green and sand green. The interesting thing about the sand green 3063 part is that the only appearance of it was:
SandGreen: 4 in 3450-1 - Statue of Liberty Sculpture (2000)
for exactly this reason, The Statue of Liberty has a green patina!!!
Of course you could use less "weird" and less expensive colors...
The sand green makes for a nice patina color as the Tsar Cannon is today, but dark orange would resemble the bronze best when it is "new", this is that color I use in my bronze swivel guns. However 3063s don't come in the dark orange color, your best bet is one of the gray(s) or stone(s) colors.