When creating an opportunity, Quote, Sample, Design Registration, etc. the header shows that the part number is mandatory, but the fields are not red-bordered. This is because Part Number is a unique field in that you add one or more Parts to an Opportunity, Quote, Sample, Design Registration, etc. It is not like other fields (Market Segment, etc.). Hence, there is no red-border around it. However any fields within the Part section will have a red border around it if they are flagged as mandatory.