On the color wheel, the secondary colors are located between the colors they are made from. The six tertiary colors (red-orange, red-violet, yellow-green, yellow-orange, blue-green and blue-violet) are made by mixing a primary color with an adjacent secondary color
As for other spacing, it's traditional to double space between the date and the salutation, double space after the salutation, and triple space for the signature. However, it's fine to only double space after the date, especially if doing so will save the letter from running on to a second page.
