When writing Esperanto, I sometimes want to use verbs which are derived from adjectives. The problem I run into is determining the transitivity of these words.

For example, suppose that I am working through a list on my screen, and each time I check off an item on the list, I do so by coloring it blue.

The verb I might want to use to describe what I do to each line might be "blui". But what is the transitivity of "blui"?

If it's transitive, I could say "Mi bluas ĝin". If it's intransitive, I would have to say "Mi bluigas ĝin". Certainly the second way is more explicit, but if "blui" is transitive to begin with, the "-ig" is unnecessary.

"Blui" is just an example - my question is actually about the general principles involved. Does "Mi bluas" mean "I turn blue" (which could also be stated by saying "Mi bluiĝas"), or "I turn (something else) blue"? In either case, why? Is there a solid rule for determining transitivity, or do we just need to always employ "-ig" and "-iĝ" to cover our bases, even if sometimes the appropriate suffix is being used where it's not necessary?

This is perhaps the only thing about Esperanto that you have to learn by heart. Every word root has a "quality", it can be adjective, noun, verb, adverb. Adjective roots, when followed by a verb ending, are descriptive, not causative. blu- is an adjective root so blui means to be blue. martel- is a noun root, so marteli means to hammer (and not "to be a hammer").

You have to learn that word for word, although in most cases it is easy to pick up.

This is what makes me say that Esperanto is not perfect (as too many enthusiast speakers use to say). Ido tried to solve this, among other things (and no, I am not a speaker of ido).

Also, the purpose of -ig- and -iĝ- is not really to introduce (in)transitivity but to specify a change in place. With -ig- you cause the change, with -iĝ- the change happens by itself or by some other party.

Mi bluas = I am blue
Mi bluiĝas = I become blue.
Mi bluigas min = I cause myself to become blue. (e.g., I paint myself blue)

mnlg:Mi bluas = I am blue
Mi bluiĝas = I become blue.
Mi bluigas min = I cause myself to become blue. (e.g., I paint myself blue)

Ah, this makes more sense. I missed the "I am X" possibility. With that being one of the possible meanings, the three forms make more sense to me now. Without it there seemed to be some overlap between the non-suffixed form and the suffixed forms, but I couldn't tell which way the overlap went.