hi Jason
the main possible reasons are:
- injectors are worn down;
- engine \ turbo suction filter is clogged;
- variable vanes (turbo) are not smoothly rotated relating to current engine load (+ impulse tube with vacuum actuator);
- late injection timing;
- possible airr suction thru primer buster pump \ fuel filter;
- possible electrical errors belong to injection circuit;
- EGR clogging;
Grey colour is overfueling during combustion stroke.
Blue colour - oil burning issue (valve seals \ piston rings)
All above is from my personal experience
Cheers \ Igor