Loving the Ultras! I never really understood the vitriol some people have towards them, they really are a stunning army on the tabletop.
I decided on ultramarines. However, I dislike the edging of each armor plate in gold.
Gold edging is more of a 30k thing. In 40k the trim of their armour denotes which company they belong to:
1st Company (Veterans): Silver according to the Codex Astartes, but Ultras tend to use Gold in all the art I see for their 1st Company
2nd Company (Battle Co.): Gold, but could be silver given that Ultras use gold on their 1st Co.
3rd Company (Battle Co.): Red
4th Company (Battle Co.): Green
5th Company (Battle Co.): Black
6th Company (Reserve Co.): Yellow
7th Company (Reserve Co.): Magenta/Purple
8th Company (Reserve Co.): Grey
9th Company (Reserve Co.): Blue
Helmet colour can also denote rank.
Red: Sergeant
Red and White: Veteran Sergeant
Pure White: Veteran
There are pretty common exceptions to the rules, and squads from different companies often work together on the field so go nuts!