The green and purple you see in your image is usually taken care of with the Red/Cyan slider but you need to remember that cyan and yellow equals green, and blue and red equal purple. Use them too if necessary. Also, the sliders are very sensitive so you might be better off manually input fractions of numbers.
Admittedly, I was very sceptical about your advice as I tried numerous times to make CA adjustment work but with no success. Yet I decided to do exactly what you said and apply one step increments of 0.05 using number buttons and pay attention to color interaction. And BINGO! It worked!!! Below is an example of my "quick and dirty" test. Looks pretty good to me. I am really glad I asked as I could not find this particular tip anywhere on the web. Thanks a lot. Is there a book or some other source you could recommend to learn this kind of details?
Practice a bit more with Aperture and you should be fine.
LOL. Glad I could help.
Most of what I have learned in life has been through trial and error. It's not the most efficient way but I like the challenge. When that fails I do what you did and throw the question out there or I look for online tips. Here is one well regarded and known Aperture site.