Go to a Games Workshop (or local wargaming hobby store) and buy a matching little pot of paint
There are a bazillion shades of white paint between the various brands. I would use an acrylic paint, because if you don't like it, you can just rub it off (as opposed to an enamel, which will make a mess to strip without damaging the original coat).
If it's old Canon white, try something like Citadel Screaming Skull. If it's a new Canon white, maybe something like Ulthuan Grey or Pallid Wych Flesh. In any case, paints like Citadel or Vallejo are in clear containers and dry in a color that's similar to what you see in the bottle, so you can just hold it beside your lens.
You can easily mix acrylic paints to get exactly the color that you want
Note that if you want it to be more than short term cosmetic (ie survive handling) you will absolutely want to prime the surface first. A (good) possibility is acrylic gesso, which will adhere well to metal. It has the benefit of being very cheap, and you can get it at most art stores.
If you prime it, paint it, and then give it a seal with a bit of gloss coat, allowing for each layer to dry, it will survive some moderate handling
I've actually fixed up a friend's 100-400 Mk1 (like you, he wanted to sell it), and it looked pretty great after