This Question has been related to this one:

Making colours from table with colour percentages/RGB-codes

In that scenario I have made sure that my layer has the right colours based on a expression. This is just because this certain dataset has "standard" we all have to follow.

The thing is this is based on a single symbol expression (aka the hexcode rgb). The colours are right, however when I want to construct a legend by the "ouderdom" layer, the colours of the legend are wrong. This is ofcourse because when I try to categorize it by that field, in the legend random colours are chosen, causing the legend to display the wrong colours.

So to clarify my symbology of the layers needs to based on the "ouderdom" field categorisation but the colouring needs to still happen the same expression (so no random colours), how do I do this?