Which dyes fail
Reds are the usual offenders, historically from madder and from early synthetic dyes that were poorly fixed. Deep indigo blues run second. Rugs woven between roughly 1870 and 1920, when synthetic dyes arrived faster than the knowledge of how to fix them, are the highest risk of all. Later commercial production is generally more stable, and modern rugs from established workshops are usually reliable. Age alone does not predict it, which is why testing rather than assumption is the rule.