I have been colouring my hair for 50 years now using L'Oreal. I used Preference for the first 45 years then switched to Excellence recently because it says it gives better grey coverage. Can't say I notice any difference.
I use half a box at a time. First time use a full box and keep the applicator bottle. From then on when doing roots put half of the 2 ingredients into the old applicator bottle and give it a good shake.
I make a parting and squeeze a little solution along the parting onto the roots massaging it a bit to spread it. I then apply it using the point of the applicator about an inch on either side of the parting going out at right angles to the parting followed by my fingers pushing through from underneath to cover my scalp evenly and I complete the all over job very quickly in just a few minutes. I also do a few streaks downwards right to the end on the dry hair. Every five mins or so I put my fingers through the dry hair and "comb" the roots up from underneath. This spreads the colour a little further down the hair shaft each time. After a few repeats I simply massage the product over my full head, leave for the final ten minutes or so then wash off.
The conditioner in the box is amazing. I towel my head dry, apply the conditioner then a shower cap and do my nails, epilate my legs, have a long deep soak in the bath, rinse the conditioner off and blow dry.
I am no beauty but have always had great hair. I mostly use a light golden blonde as that was my natural colour and I am regularly complimented on my hair. In winter I sometimes go one shade darker covering roots and putting on lots of full length streaks. My hairdresser, who understands that I prefer to do my own colouring at £4 a time has told me that home colouring is far gentler than the "professional" products.
I have never noticed build up. When you think of it the hair I am colouring now has only been around for four years or so as hair grows really rapidly (just thinking of the amount on the floor from that first cut after lockdown)
It is seriously easy to do, really inexpensive, and gives great results.