However, colour is unlikely to become popular with mainstream poets because (except for those who are synaesthetic) it's not a core property of language or even of writing. Sound is, though some poets claim they pay no attention to the sound of their work. Punctuation is less central (though pauses are natural), fonts and linebreaks less central still.
The significance of these effects (along with creative misspellings) is demonstrated by the way editors treat them. Sometimes (as in Finnegans Wake) misspellings are an important part of the work, but in practise Americanisms or single mistakes are likely to be "corrected" by UK editors. The house style will take precedence over the author's font choice, and titles are sometimes put into upper case too. Linebreaks are usually retained (though a long-lined poem can be broken by a magazine's insistence on a two-column layout) because they are considered expressive, part of an author's voice, but in many ways they share the problems exhibited by colour, use of multiple fonts, or Joycean spelling. Those spurning these latter effects probably don't feel that they're missing out on opportunities to enrich their works. There's little mainstream tradition of using them - they're seen as attention-seeking gimmickry. Freeform linebreaks are different only in that a tradition exists albeit in different contexts. In other respects linebreaks are as peripheral as fonts and colours; capable of expressing nuance, but essentially a visual effect.
Whereas punctuation derived partly from transcribed speech, linebreaks are a consequence of the rise of the written word, and were used in grafitti, gravestones, advertising and formal verse before freeform poetry appeared. In freeform unlike formal poetry there are no fixed places to where linebreaks should be used; each linebreak is a deliberate act (the prose linebreak seldom used) and readers will seek to explain the choice. Adding linebreaks is an easy way to make readers work harder without the author needing to work. Freeform poetry could use prose-linebreaks as the default rather than the all too common end-of-the-clause linebreak.
Had there been no metrical verse, and colour was easily available would freeform linebreaks be as dominant over colour as they are now?