Put the rhubarb and water in a high speed blender like a Vitamix, or a food processor and process until very smooth.

Press the puree through a fine mesh sieve so the rhubarb juice comes through, leaving the solids. Use the back of a spoon to as much through as you can. You'll need 1 cup of juice.

Put the juice into a saucepan and stir in the sugar and lemon juice. Add the whisked eggs and whisk everything together well.

Bring the mixture to a simmer, stirring or whisking constantly, over medium to medium high heat. You can add a few drops of food coloring at this point if you need it.

Once the mixture comes to a boil it should be slightly thickened and should coat the back of a spoon (it will thicken more as it chills.) Remove from the heat and add the butter, a bit at a time, whisking in to melt each piece before adding the next.

Pour the curd through a mesh strainer to remove any bits of cook egg, then fill your jars. Let cool to room temperature before covering and refrigerating overnight.

Use the curd within 2 weeks.