Tripped me up too. I didn't expect the need. Then I thought maybe there was some restriction of which blocks could go where, or which blocks needed to be placed first. The first letters I placed, by coincidence, worked fine, so that contributed to my confusion.
I thought color would be assigned automatically as soon as possible. So if there was an ABCD block and a ABEF block, then you could place an uncolored B, and as soon as you placed an F in the same block that block would get ABEF color.
Wanting to just type and have the game do the right thing is a very reasonable expectation, and auto-assignment is an interesting idea. It might need to be a one-way street to avoid unpredictable cascades though, such that you can go from "any letter A" to a "concrete letter A in that specific group" automatically, but not from a "concrete A" to "any A".
Another approach (what I assumed would happen) is to have each group's logic independent, and not cascade. So it doesn't matter if I've already assigned the ABC group, typing a single A keeps the ADE group white, up to uniqueness.
I think the biggest issue with group assignment is the fact that you just can't type while the wrong group is selected. I don't care what tools you have for narrowing down letters, if I see a solution in my mind, I should be able to type it out like I could on a piece of paper. Assignment feels more like adding flags in minesweeper (optional, helpful) than part of the solution.
