Heat oven to 325.
Prepare a loaf pan by greasing it or coating it with baking spray.
In a small bowl, mix together flour, seeds, baking powder and salt.
In a small cup, mix together milk, juice, vanilla, rind, and optional food colouring, and let sit to sour slightly.
In a mixer, beat butter and sugar until fluffy.
Beat in eggs 1 at a time.
In alternating batches, stir flour mixture and milk mixture into butter mixture.
Spread into the loaf pan.
Bake for approximately 1 hour, until a skewer comes out clean.
About 15 minutes before the end of baking time, make the grapefruit soak:
In a small pan heat sugar, rind, juice, vanilla, and optional colouring, stirring just until the sugar is completely dissolved.
Remove the soak from the heat.
Right after the loaf comes out of the oven, pierce the loaf with a skewer in 12-24 places.
Immediately spoon the syrup over the hot loaf.
Leave the loaf in the pan and cool completely on a wire rack.
For even more concentrated flavour, wrap loaf tightly in plastic and refrigerate for 12-24 hours before icing.
For the icing, placing icing sugar and rind in a bowl and mix.
Add vanilla and mix.
Add juice by the tablespoonful until icing becomes thick but spreadable.
Add optional food colouring if desired.