The poem is ...
I tried to solve a sudoku
Without a clue of what to do
I guessed and queried quite perplexed
Eventually I got quite vexed
I finally solved the tricky quiz
And now know what tag fusion is
The completed sudoku puzzle looks like this
- hopefully no errors made in the transcription from the original text version

Original text version solution - this is what I used to solve it:
CulOZ EpKhr nTymA wqIvx bFDSG
XqiMS ozlBF rKhPD aueGY vCwTn
dbkpt sQyVc OZUwg hnLfr MAXeI
YAEVN tgdIW fBsxl pKmZC hoQRu
rGWhF aNmUx EqVcI bStod KyLpz
uCmwE yohzn trfvp QAbkL XIsgd
qxpKo gEiFu BNdaw sHyrv ClTZM
BdhZl PsXkv mOIGu NWfCt EqYar
aYtsi qrcwd zexlh upomg NkfBV
gRfnV lTBMa QCKsy eixDz pWoUh
sfABQ nltGe kmzov idRXP wUhyc
zEGrH fASOm lUtyq kBCnW dXiVp
OWyDC RkqXZ PfgIn TeVhU lbAMs
pTVIx hwUdY cSABe LmgQo rNZFK
mknlu IBVcP xdWhr YfzaS gTEoQ
flxap kYgrQ uiMNb dzstH OVCwE
eVdQR wUonl SyPKz Mcabi THgxf
WhsgB cvEti DArQo fxuln zPmky
tiOCY BMzps VhEfX gRqwK adunL
KnZuM xDfah GwLTc ovPYE isRQb
lmueA zhryO iVnds xTKPq FGBCw
iPrtG uxweb yLCZk VOHsf QMnda
nsCfk dipQT hxBUm ZgwEA yrVlo
HoBXw vcnlK AGqEf RyDuM szPit
vZQYD mfasg wpoRt cLniB uekHx
Uppercase are letters that were supplied; lower-case were added while solving.
It should be noted that the sudoku is not fully constrained; I was left with a handful of naked pairs & triples that couldn't be resolved without just arbitrarily picking a letter somewhere and working with it.
OYZ at col 9 row 6&21 / row 21 col 6&9 / row 2 col 6&7 / col 7 rows 2&6
BS at col 22 rows 13&20 / col 25 row 13&20 / row 13 col 22&25 / row 20 col 22&25
For the colored squares ...
Reading each color in order as it appears in the puzzle, you get:
$\begin{array}{lcl}\mathbf{blue}&&\text{EveNtuallYigotqUITevEXed}\\ \mathbf{violet}&&\text{AndnoWKnoWwhAtTAgfuSiOnis}\\ \mathbf{yellow}&&\text{wIthOutaclUeOFWHattodo}\\ \mathbf{green}&&\text{IGuessEdANdqueRiedQuItEPERPlexED}\\ \mathbf{indigo}&&\text{iFINaLLysolvEDThETricKyQUiZ}\\ \mathbf{orange}&&\text{asUdOkU}\\ \mathbf{red}&&\text{ITriedTOsOlve}\\ \end{array}$
Adding spaces and putting the lines in their spectral ordering ROYGBIV(thanks OP!) gives the poem as presented above.
How I solved it
I solved what I could via normal Sudoku rules. When I got stuck, I look at the text of the messages and guessed what letter or entire word choices seemed highly likely to complete the poem, and used those to fill in their colored squares in the grid. Then back and forth between those two methods until I got the whole thing done.