Trevor Griffiths, in his My World of Old Roses Volume 1, 1983, lists Colonial White (p.142) and Sombreuil (p. 143) as different roses. Unfortunately he doesn't include a photo of Colonial White, but describes it as "A vigorous-growing hybrid with healthy foliage, long canes and large prickles. The flowers are over 75mm across and a dull white, quartered and very fragrant. They open flat and are recurrent."
His photo of Sombreuil matches what is grown under that name. he says "A vigorous climber to 5m. Creamy white flat very double flowers about 100mm across which are deliciously scented."
In the introductory chapter he says his descriptions are based on observing the plants in his own garden.