Phone numbers in iPhone's Notes are automtically hyperlinked and hard to see
One option on your iPad is after to type in your number tap the Aa button on the left side over the keyboard. This won't allow you to change the font color but you can change the font size by selecting Title and make it bold. This increases the visibility by a lot on my iPad. You must do it immediately after typing the number in.  Here is the location of the formatting buttons.