I am assuming you are writing perhaps a first-person shooter type game.
If you use a Sprite, parented to the camera (or a pivot-parented to the camera etc - so that it stays in view and moves with the camera) - if the sprite is a white crosshair on a black (so it's transparent) background - EntityColor will affect the colour of the sprite.
Otherwise, I don't know if WritePixelFast is any good? ( I haven't used that command really!)
If you use a Sprite, parented to the camera (or a pivot-parented to the camera etc - so that it stays in view and moves with the camera) - if the sprite is a white crosshair on a black (so it's transparent) background - EntityColor will affect the colour of the sprite.
Otherwise, I don't know if WritePixelFast is any good? ( I haven't used that command really!)