I've revisited the guide recently and have a few more comments.
When we evaluate the color of spells, the efficacy is only one consideration. The 2nd most powerful spell in the game might sound like something that should be gold to us - but it might actually deserve to be red. Why? Let's say both were 9th level spells (of which we only get to cast one) and we'd always elect to use the most powerful over the second most powerful. Why prepare the second 9th level spell when we'll never use it? Why bother selecting to learn it? This is an argument as to why the presence of a gold 9th level spell (wish) makes every other 9th level spell green at best - maybe purple.
So, when we apply color to a spell, I think we need to consider:
* Whether it needs to be prepared while adventuring to be useful (rituals, simulacrum, clone, etc... do not);
* In what situations we would prepare it
** Are there levels where utility phases out - like sleep phases out,
** Would we work it in or out based upon situational benefits - wall of force is great unless you're fighting underwater where people can swim around it, and
** Would we always prepare it - like shield?;
* How often will I use slots on it when prepared; and
* How powerful it is.
To me, any spell that I'm always going to have prepared once I learn it and will cast using multiple spell slots over an adventuring day; or that I've often use without having to regularly prepare are gold - even if they're not amazeballs (technical term) in power levels. Other spells might also be gold based upon a totality of circumstances. This assessment would upgrade a lot of rituals to gold.
There are 40 light blue and gold non-ritual spells on your list. A few of those spells (contingency, clone, simulacrum) are not ones we prepare daily, but most are. A 20th level wizard with a 22 Int and Spell Mastery included has 28 spells they can prepare. Opportunity cost says that a lot of those sky blues and golds are not going to make your often prepared list - we'd be able to prepare only about 2/3 of the available ones at any given time. Some of those are spells we'll conditionally prepare, but they're awesome at the levels we'd prepare them or in the situations we'd use them. So, if we say that we're cycling between these spells that number of sky blue and gold make sense... but only if every other spell is purple or lower. The more dark blue and green options we have - which by definition are the minimum tiers where we'd consider them as a selected option - the less sky blue and gold we can have if they need to be prepared during adventuring to be useful.
To me, based upon the description of the categories, this is how I'd rank spells:
Gold: Rituals I will use often, spells I will cast while not adventuring that are very useful (clone, simulacrum, etc..), and spells I need to prepare on adventures that I will always prepare once available, although they may get 'phased out' after a while. There should be very, very few of these if they require preparation while adventuring to use them.
Sky Blue: Rituals I will use once or twice a level, spell that I will cast while not adventuring that are fairly useful, and spells I need to prepare on adventures that many character builds will prepare most of the time. Based upon the limited number of preparable spells, there is really only enough room for 4 or 5 sky blue or gold spells that require active preparation to be useful in level 1, 2 or 3 per level for levels 2 to 5 and no more than 6 total for levels 6 to 9. Across 9 levels - excluding rituals and spells we don't need to prepare while adventuring for them to be useful, there should be no more than about 20 spells that are sky blue or gold. Having more violates the definition of the colors.
Blue: Rituals and spells I do not need to prepare while adventuring that are of limited use fit here. So do spells that certain builds will love, but others would rarely, if ever, prepare; spells that phase out quickly; spells that have alternatives that provide similar benefits at a lower level; etc... There should be a handful of these spells at each level - but not if we have a gold option at a higher level spell.
Green: Any other ritual or spell that I do not need to prepare while adventuring should fit here unless totally useless. Spells with conditional use or a very short useful life cycle that must be prepared go here as well. This is the highest tier where any number of spells could fit.
Purple: Totally useless non-adventuring spells and rituals fit here (basically, a seemingly useless ritual is better than a seemingly useless non-ritual). So do prepared spells that have very narrow bands of use.
Red: Junk.
Also, a discussion of non-wizard spells available via dip into another class would be good in a multi-class area. A one level dip into cleric gets you first level cleric spells - and that means access to cure wounds that you can cast using high level slots. That deserves mention. First level spells that ramp up well can be very worthwhile - and having the capability to use up a huge number of slots to heal the party up is sometimes worth it.
