This is another DUO easter egg -- a hidden gem of a well-known phrase. In this case, it's a movie title, How Green Was My Valley. For fans of old movies, it's (still) worth watching!
What's your reasoning here?
When we say "What a beautiful baby!" we say "Kia bela bebo!" so it would make sense that "How green!" would be translated to "Kia verda!". (Or at the very least "Kiom verda!" ("How much green!".))
But "Kiel verda!" for "How green!" just seems wrong to me. Using kiel in "Kiel verda!" is more like saying "In quite a manner it is green!" (remember, the -el correlatives denote manner).
In contrast, "Kia verda!" would mean "Such a green!" which better conveys "How green was my valley!".
Wouldn't you agree?
"Kiel" also has the following function: http://bertilow.com/pmeg/gramatiko/kvantaj_vortoj/grado_kvanto.html#i-m51
It doesn't make sense to use "kia" in the sentence of this discussion. It would have the following literal meaning:
"Kia verda estis mia valo" = "What a green my valley was" (?!)
If you want to say "Such a green valley", that would be "Tia verda valo".
Regarding the use of "tiom", it's worth bearing in mind the so-called "Tiom-kiom-ismo": http://bertilow.com/pmeg/gramatiko/kvantaj_vortoj/grado_kvanto.html#i-7ko