Both the convoy symbol's shape and color are dependent upon the designated lead ship. As stated, if it's a sub, the convoy symbol is a circle. If it's a neutral vessel, the convoy symbol is green.
You are far more apt to see the latter than the former. Most convoy symbols will be red squares (lead ship is an enemy-flagged merchant). Most convoys before Dec 07, 1941 will contain both enemy (red) and neutral (green) vessels.
On very rare occasions, you'll encounter a small, totally neutral, convoy led by a single escort. These are usually found coming out of the Straits of Gibraltar into the Atlantic.
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Raptor
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