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Colera for cholera is not surprising, as post-classical Latin often omits the letter h, particularly from the digraphs ch and th. Most of the Medieval and Neo-Latin dictionaries in the Database of Latin Dictionaries list colera as a variant spelling of cholera, and Brepolis' Library of Latin Texts has about 100 instances of this spelling.

I can't find a parallel for forter, but fortiter would make sense in this context. The shape of the T, which is different from all the others in the text, is probably meant to indicate an abbreviation. I'm not seeing any other uses of a minuscule-like T as an abbreviation mark in my brief search of similar British Museum objects, but a mark above a letter often indicates an abbreviation in Latin paleography, so we should read fortiter. This interpretation is strengthened by the fact that with fortiter the sentence is an elegiac couplet, while forter would not work with the meter. The other three prints in the series are also elegiac couplets: melancholy, sanguine, and phlegmatic.

Onme must mean omne. It's a very rare misspelling, but the Library of Latin Texts brings up a couple parallels for this: onmibus appears in a 12th-century "Chronica Hispana" and in the histories of Gregory of Tours, and a 4th-century biblical gloss has onmes, all in places where omnibus or omnes would make sense. (I'm not familiar with the other two texts but Gregory of Tours is a Merovingian author with notoriously non-standard Latin.) Interestingly, the phlegmatic print from this series also has a metathesis of M and N, writing acunime (not a word) for acumine 'point'.

The British Museum website agrees with me on both fortiter and omne—not that we have to trust the BM's transcription, which also says effecten (not a Latin word) where the text clearly says effectu, and which is missing entire words in two of the other prints from the series. I mostly agree with your translation, but suo modifies effectu, and que is the feminine relative pronoun (classical quae). The syntax is weird, but not wrong. There are two relative clauses, one inside the other, and both relative pronouns are in the middle of their respective clauses, so that colera is the antecedent of que even though there is another relative pronoun in between them.

So I would transcribe it as: Tristes at colera sunt hi qui semp(er) abundant / fort(it)er effectu que movet onme [omne] suo. I would translate it as: But sad are they who are always full of cholera, which moves everything strongly by its force.