True Blue - Loyal and unwavering in one's opinions or support for a cause.
'True blue' is supposed to derive from the blue cloth that was made at Coventry, England in the late middle ages. The town's dyers had a reputation for producing material that didn't fade with washing, i.e. it remained 'fast' or 'true'. The phrase 'as true as Coventry blue' originated then and is still used (in Coventry at least).
There are other theories as to the origin of 'true blue'. For example, the representation in paintings of the Virgin Mary in blue clothing, the purity of lineage of the Spanish nobility, or the blue aprons worn by butchers. These derivations are unlikely as they aren't supported by documentary evidence that link them to 'true blue'.