To be fair you do have a good Point and I agree! And its quite true what you mentioned.
Also I do think they could add more Solid coloured blocks,
To fit the whole spectrum for colours blocks, perferable different shades of things, (If it isnt too hard ofc)
as for the Kelp, Thats true I kinda forgot about that LOL, However I rarely see people often go into the ocean, to live (or need food) most of the time which could also been an issue about the kelp, and probably because of that I dont think people would rather use kelp since you often have to "Dry" aka cook it for more coal than its worth, while you can simply cook food for cheaper. which propbably discourages people to collect it, and thus isnt used quite often sadly. However kelp is also a "Unique" type of block in a way? I mean While its a pretty block I think cause of how dark greenish black, that it struggles to fit into builds nowadays aswell which also leads to its unpopularity too it.
All In all however I completely agree with you're statement! I'd think it'd be cool to have a "Paint brush" tool that you could apply dyes into and then apply it too blocks like Wood for example, or cobble stone It'd give more diversity into the aspects of builds with colours you couldn't normally obtain. OR they could make it so you could have a more Diversity of dye mixes like Mixing red + green, then mixing that colour with something, then you'd be able to use it (THE ONLY DOWNFALL of this idea however would be the amount of codeing it would require for them to even make that a feature so who knows?) Anyways thank you for commenting! and lettng me know you're Opinons :)