There is an index to Smallburgh parish registers on the Norfolk Transcription Archivehttp://www.genealogy.doun.org/transcriptions/index.php
There are also a couple of photos of the church there (from home page, select 'parishes' > 's' > 'Smallburgh' > 'photos' The thumbnails can be enlarged by clicking on them).
[William  seemed to have added an "e" in his name when he went to America. I don't know if that's common?]
Spelling of surnames did not become finally fixed until the beginning of the 20th century, prior to then you will find variant forms.
Emigration to Orleans County and that area of New York State was quite common, large numbers of agricultural workers used assistance granted by the Poor Law Amendment Act 1834 to gain passage - the records which survive are held in The National Archvies at Kew in document class MH12 and can be very usedful.
Check 19th century census returns for New York State and you'll find lots of common Norfolk surnames.
Your mob emigrated after the 1851 census when they are recorded:
HO107/1808 f211 p18
Chapel Road, Smallburgh
William COLMAN, hd, mar, 51, ag.labourer, bn 'Not known' Norfolk
Mary Colman, wf, mar, 53, bn Ashmanhaugh
William COLMAN, son, unm, 24, ag.lab, bn Smallburgh
Rebecca Colman, dau, unm, 18, bn Smallburgh
George Colman, son, unm, 12, ag.lab, bn Smallburgh
Harriet Colman, dau, 10, scholar, bn Smallburgh
Margaret COLMAN, mother, wid, 86, pauper, bn Worstead,
I would suggest that William was baptised at Smallburgh in 1797, the son of John + Margaret (Late PORTER)
The couple probably married at Sloley in 1790, when I have the bride's surname as POINTER.