I have read on a post that phosphate bolts & nuts would be dyed with opaque ink and zinc plated would use transparent ink is this true?
Don't know that I agree with that suggestion or that all of the hardware suppliers even did the exact thing (specific dye product) as other suppliers. It may be governed more by the specific part and the supplier.
For example the brake line clips for some years appear to have an opaque finish while other years they don't always seem to look as "pasty". You can make out the uneven coating (dye job) along the top of the picture where there are runs and build-up in the lower surfaces and bends of the metal where it collected during drying
I've always used the red layout dye like the Dykem product shown below, but as mentioned in an older thread they have reformulated some of their products to an opaque base so you might need to hunt a bit or experiment. Red layout dye might be a different formula
Don't know that it needs to "stand out" as many original examples don't especially over phosphate