Note: in the latest versions of ColorProof and OpenColor, the db3 fallback is automatic. This means you only need to define a Map to db3 rule if you do not want the colour to use an OpenColor ink at all. Any other missing colours without a Map to db3 rule will automatically map to db3 anyway.
When using db3 spot colors in combination with an OpenColor job, a rule has to be defined first in the Channel Mapping, when using db3 and OpenColor inks in the same job.
When you are printing a job with OpenColor, it has several colours inside it, for example:
Often you will use a mixture of inks from OpenColor and db3:
C --> Ink from OpenColor
M --> Ink from OpenColor
Y --> Ink from OpenColor
K --> Ink from OpenColor
Pantone 1 --> Ink from OpenColor
Pantone 2 --> Ink from OpenColor
Cutter --> Ink from db3
Layout Guide --> Ink from db3
When you are using an OpenColor project (Dynamic or Static MXN) in a job, it will not automatically find the inks you wish to be taken from the db3. It only searches the OpenColor project inks and if the colours are not in the project (like Cutter, Layout Guide, etc) - a 'spot color not found' error is shown.
OpenColor projects can have all of the Pantone libraries, so it's only custom db3 colours that are the concern (e.g. cutters, golds and the like).
There are two options to work around it:
1) Manually map or define the colours in the job properties as and when they come in.
2) Set up a Channel Mapping rule set for all your custom db3 colours that you will use with OpenColor, and activate this rule set in the workflow. In a Channel Mapping rule set, you can list each colour and set it to 'Map to Db3'. This means it will automatically map the colour to the db3 version every time.
E.g. if you have these rules defined in your rule set:
Cutter --> Map to Db3
Layout Guide --> Map to Db3
When you send a job to ColorProof that uses an OpenColor profile, it will not error with 'spot color not found' and instead find the db3 definitions automatically.
Please see below a screenshot which shows the location of Channel Mapping.