If "bold" refers to courage or confidence:
* Timid
* Shy
* Cowardly
* Fearful
* Hesitant
If "bold" refers to a strong, striking appearance:
* Subtle
* Bland
* Muted
* Pale
* Delicate
If "bold" refers to a statement or action that is daring or unconventional:
* Cautious
* Conservative
* Traditional
* Conventional
* Prudent
Ultimately, the best antonym will depend on the specific meaning of "bold" in your context.