I wouldn't put a piece of angle over the corner as this would be too thick. If you are having trouble hiding the seam and you don't have any good filler, then I would recommend a thin piece of paper.
As for the trim color, that depends somewhat on the era, location, and what other colors were used. A newer house in a nice area when times were good may have used several different trim colors.
An older home in a not-so-nice area, or one in poorer times would usually be painted all one color, typically white.
One thing that bothers me about these kits is that the window sashes end up being too far out of the structure. While the window trim would be outside of the siding, the sashes should be inset, and thinner.
That attic window looks like it was made with 4 inch x 4 inch wood posts.