Color dots on my calendar are not much of a choice because I have trouble seeing the lime green dots. How can I change this color?

In the Calendar app touch on 'Calendars' at the foot of the screen. This shows a list of all your calendars.

Find the calendar for which you want to change the colour. To the right of the calendar name touch on the i in the circle. In the next screen you will see an option to select a colour.

Make a selection, touch on the Back label to return to the previous screen and then on Done to lock in your choice.