how is a home lizard's droppings mostly black + partially white??

Answer 1

some is fecal matter, and some urine, i understand, much as birds', with which lizards share common ancestors.

Answer 2

Almost exactly like a birds droppings, a lizards excrement is both feces and urine. the black is the processed food matter, and the white is used minerals and excess water.
What kind of lizard is it by the way?
If it is an iguana, I'd like to know what you are feeding it, because they need a special lettuce to get proper nutrition.