It is not common in comparison to inks intended for high contrast, but you can buy ink for fountain pens that is white or off-white. (If the off-white isn't pale enough, it can be diluted with water to further lighten its color.) It's also possible to buy ballpoint or paint pens with white ink/paint.

Asking at your local arts & crafts store is the fastest way to find a good source of such implements.

There are many possible arts & crafts uses for white ink, so it's definitely possible to buy pens with white ink/paint. Look for a "gel" pen with white ink, and it should meet all the requirements that you have outlined.

Asking at your local arts & crafts store is the fastest way to find a good source of such implements, but there are also plenty of online suppliers (here is a Guide to Choosing White Gel Pens and Markers from one example).

It is not common in comparison to inks intended for high contrast, but you can buy ink that is white or off-white. If the off-white isn't pale enough, it can be diluted with water to further lighten its color.

It is not common in comparison to inks intended for high contrast, but you can buy ink for fountain pens that is white or off-white. (If the off-white isn't pale enough, it can be diluted with water to further lighten its color.) It's also possible to buy ballpoint or paint pens with white ink/paint.

Asking at your local arts & crafts store is the fastest way to find a good source of such implements.