The main difference between "promising" in "promising result" and "washing" in "washing machine" is that "promising" is only an adjective that has hardly any verbal force or meaning. In general, -ing forms used as adjectives like surprising, interesting, boring, breathtaking, amusing, etc. do have a verbal meaning ("an interesting book" is a book that interests the reader), but that is not the case with "promising".
In "washing machine" -- which is a compound noun -- "washing" can be interpreted as a gerund (a machine for washing), or as a present participle (a machine that washes). The same test may be followed with most compound nouns of this kind: in "walking stick", "walking" is a gerund (nominal) because the noun can be understood as "a stick for walking" (NOT a stick that walks), and the same applies to swimming pool, but more often than not the -ing form in compound nouns can be interpreted either as a gerund (nominal) or as a present participle (adjectival), as in "washing machine" or "cleaning products" (products for cleaning, or products that clean).