just noticed something... i have to disagree with your assessment of silent image.

what can you do with it that you can't do with minor illusion?

1) combine it with minor illusion to get both sound and sight effects.
2) cover 27 times the volume
3) create an illusion of a creature
4) create an illusion of "some other visible phenomenon" (offhand, this would probably include, say, fire, which is not an object or a creature per se).
5) move the illusion.
6) use it at double the range.
7) arguably have the image move at all (minor illusion doesn't specify it can't, but silent image and major illusion both make special note of the fact that in addition to being able to move the illusion's location, you can also make the image move appropriate to the change in location, such as making a creature appear to walk).

now, there is *definitely* something to be said for the ability to use minor illusion at-will, plus the fact that minor illusion is only competing with other cantrips whereas silent image is competing with one of your extremely precious 15 spells known. not saying your rating is inherently wrong (though as with most illusions i consider it more situational on DM than what is happening in game), but it is definitely a substantial improvement on minor illusion in a variety of ways.

likewise your comments with major image. it isn't just bigger. it has better range, allows you to combine sound and image 4 times as far away as the silent image/minor illusion combo, and covers an additional two senses (fire is a lot more convincing when it looks, sounds, smells, and feels hot like fire). lastly, at higher levels you can make the image permanent and concentration-free. which means that, for example, you can expend a level 6 slot today to create an illusion of a fog cloud, introduce everyone in your party to the fake fog cloud so they can see through it, and spend your actions for the rest of your life until someone dispels the fog cloud completely to start off every battle with a 20 foot cube of fog that you can see through but your enemies can't until they get close. heck, if you're willing to spend some prep time, you can technically learn the spell at level 11, spend a few weeks of downtime stockpiling major illusions, and then swap the spell out as soon as you hit level 12 and come back for a fresh copy of the illusion if anyone actually does blow a dispel on it (of course, being able to do something is not the same as it being a good idea for you to do that something; your DM may get slightly annoyed if you actually unlearn the spell).

again, neither spell is necessarily an automatic selection for a sorcerer, or even a good selection for a sorcerer most of the time (obviously, if being an illusionist is your goal, you want them, but not necessarily for a typical adventuring sorcerer). extremely limited spells known may even keep these spells rated purple for most campaigns. but those spells are in fact quite an improvement. the only reason i wouldn't put them as dark blue is the extremely crippling spells known limitation WotC felt necessary to impose on sorcerers.