I'm preparing a document for print and I am in the preflight process. One of the errors is the colour issue shown in the image below.
“Registration color”: Black
How can I fix this?
Simple answer: Don't use the [Registration] swatch in InDesign*.
Registration black is 100% of all inks. It should only ever be used in registration marks used to reference the alignment of the different inks or plates used. Don't ever use registration black in your artwork. Ever. Your printer will hate you (the man and the machine).
A true CMYK black is only 100%K and nothing else, whereas a "Rich Black" is made up of 100%K and some other inks, but never 100% of all inks. For a more detailed discussion on which blacks to use in CMYK printing you can read through this previous Q&A: What kind of black should I use when designing for CMYK print?
* Ever. There's no real need for the [Registration] swatch to be there, it looks exactly like the [Black] swatch and you can't delete it. Move it to the bottom of your swatches and ignore it (unless you're actually creating your own registration marks of course).
Somehow you have applied the registration swatch. Easily fixed . . . use find-and-replace.
In the find dialog, select "object" then specify the attributes to find (registration used as stroke, etc.) and specify new attributes (e.g., replace stroke with black).