Complementary colour generator
Nearest named colour: royalblue
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Gives the hue 180 degrees from yours. Complementary pairs produce the strongest contrast of any two-colour scheme, which is exactly why they are easy to overuse.
How to use the complementary colour generator
A complementary pair used at equal amounts fights: two colours of similar saturation directly opposite each other create a vibrating edge that is genuinely uncomfortable to look at. The convention that makes them work is a heavy imbalance; roughly 90 percent of one and 10 percent of the other, with the small amount used as an accent. That is why "red and green" sounds garish and a green room with one red chair does not. It is also worth knowing the wheel here is the RGB hue wheel, not the red-yellow-blue wheel taught in art classes, so the complement of blue is yellow rather than orange.
Questions
The hue directly opposite on the wheel, 180 degrees away.
At equal amounts and similar saturation they vibrate at the shared edge. Use roughly 90 to 10, not 50 to 50.
The RGB hue wheel, so the complement of blue is yellow. The red-yellow-blue wheel from art class gives different answers.
Not necessarily. A palette generator arranges hues; it says nothing about contrast. Check any text-on-background pair against the contrast checker before shipping it.
Hex for anything going into a design tool, HSL when you want to adjust lightness by hand, and OKLCH when you need two colours to look equally light; it is the only one of the four where that number means what you expect.