Wash the limes thoroughly before beginning.

Chop off the blossom and stem end from each lime and discard.

Slice the limes into very thin pieces using a sharp knife or mandoline. Once sliced, chop each slice into quarters. (Alternately, quarter the limes and then pulse in a food processor until the pieces are evenly chopped but are not mushy.)

Combine 8 cups water with the processed limes, stir, cover, and let the mixture sit overnight.

In a large saucepan or jam pot, bring the lime mixture to a boil, and continue to boil uncovered for 40 minutes until the peelings in the mixture have become soft.

Add 6 cups of granulated sugar gradually to the lime mixture as it boils, continuously stirring to ensure the sugar is dissolved.  Add up to 1 more cup of sugar to taste if needed, continuing to stir until sugar is dissolved.

Remove the mixture from heat, cover, and let it sit for 6 hours or overnight.

Prepare jars or containers for the jam, and prepare a hot water bath if you plan to can the marmalade.

Return the lime mixture to the stove over high heat, bringing it to a boil.  You may need to add a little water if the mixture has set up too firmly overnight.

Stir continuously to prevent scorching.

Using the cold plate test, cook the marmalade until it is firming up but still has a slight syrup consistency.  Alternatively, you can cook the marmalade to a temperature of 216 to 218 degrees Fahrenheit, and then test the marmalade using the cold plate test.

When the marmalade has cooked long enough to pass the cold plate test, taste the marmalade and add freshly squeezed lemon juice to taste if desired. If using green food coloring, this is when you'd add a drop or two, if desired.

Remove the marmalade from the stove and let it sit for 10 minutes.  Finish getting your jars or containers ready.

Stir the marmalade mixture well so that the contents are all evenly distributed, then ladle or pour into jars or containers. If canning, leave 1/4 inch headspace. If freezing, leave 1'' headspace, and be sure to use freezer-safe jars.

Preserve the marmalade by using a hot water bath canning method, or after the marmalade has cooled, keep the marmalade refrigerated or frozen until ready to eat. 

If canning, process the jars in a hot water bath canner for 10 minutes (or 15 minutes if above 6,000 feet in elevation). Remove the jars from the canner and allow to cool for 24 hours before checking the seals. Store any unsealed jars in the refrigerator for immediate use. Properly canned and sealed jars of lime marmalade will maintain peak quality for about 18 months on the pantry shelf. Refrigerate after opening.