There's two sets of logic, subject to the following over-arching constraint: There are 5 main tiers, and a 0th tier for certain classes of items/entities that have 6 entries.
The sets of logic are:
Traditional:
For most things, tiers will start with tier 1 and count up. For the entries with 6, they start at 0 and count up. Everything ends at most at tier 5.
Progression:
Tiers for structures/items (not equipment so much) in general correspond to a loose grouping of materials that represent increasing level of difficulty/complexity/game progression.
The reason Tier 6 (orange by default) exists is to handle 3-4 edge cases in Angel's mods when used with Extended Angels and progression. In a standard Bobs / Angels / Bobs+Angels, Tier 6 does not appear.
I don't think I am going to change how this works, Dark Angel. The gray/white/0th tier is a Bob-native construct and for equipment, which consistently has 6 entries and doesn't really correspond to a material-tier, having the final tier be green for consistency is important.
Drills may start with yellow, but you're forgetting there's a Burner mining drill. 0th tier is pseudo-burner tier/phase. There are 6 assembling machines, and the first is 0th tier. The basic belt is 0th tier. Burner/steam inserter are 0th tier. There's a number of other examples outside of equipment.
I understand...
It's just hard for me to understand.
I'm used to seeing that everything that has levels starts with yellow. For me the 0 level with his white color to see is not very logical, yes I understand that this is in the very fashion made so that would be 0 level ...
But still would like to have a single color palette for everything.
Drills, assembly machines, oil rigs, tanks, and so on... everything starts with yellow...
That's in the traditional way.
I tried playing with the progressive... it didn't work for me.
I needed a division specifically by level of promotion, such as from 1-6 and the like, no 0 level.
I put a lot of similar mods, but it was your mod that did with the levels of buildings and some items - such as repair packs, exactly what I expected from them. A clear division in increasing levels.
And as you wrote... it would be better to do 1-6 with the last orange color. And another problem is that level 0 does not draw any circles or any other definitions, purely icons, while yellow - red, etc draw additional icons on top - for the first level - 1 yellow circle or triangle, for the second level - 2 red circles or triangles, etc...
I still hope you add a feature to remove the 0 level.
It would look much better.
This is my personal opinion.