The grout line is the distance that separates one tile from another and is a necessary element because it permits dilation of the floors and natural settling due to structural movements, humidity, and variations in temperature.
Despite being necessary, not everyone likes the contrast created between the colour of the grout lines and the colour of the tiles. If this is the case, one should look into rectified flooring (tiles with perfectly straight edges) which makes it possible to leave a much narrower grout line, starting at just 1mm.
The colour of the grout lines is just as important to reducing contrast and creating the impression of a continuous surface, suitable to modern spaces.
Some advice on choosing the colour for grout lines, so as to achieve a uniform result:
Choose a colour that is slightly lighter than that of the tiles: over time grout lines tend to get dirty and become darker.
When using a product with many different shades, like wood effect porcelain stoneware, it is better to use a neutral colour for the grout lines, one that picks up on the most dominant colour in the tiles. In this way the laying of the tiles will be uniform and the grout lines will naturally blend into the flooring.
Light colours get dirty more easily, especially on the floor and in spaces prone to soiling, like the kitchen, the entrance, and the garden.