I'd give it as you'd have it on official documentation (if you were applying to anything in Japan(ese). Much easier for them to infer a shorter version if it's appropriate, then to go back カラー initially, then have them wonder who カルメリツ is.
To be honest your name isn't so "bad" - my last name last both elongated vowels and double consonants - even then, most people get it right the first time. I'd say 95%.
と申します。スミスて言ういいよ。 <-- Although this is common in English, I rarely hear it in Japanese (and not with two difference levels of politeness) unless it's a teacher wanting their kids to call them by their first names instead of their last. Not out of the question, just rare. A shortened name/nickname is a sign of closeness in most cases, and it is not something you can presume to have the other person use (if that makes sense). Again, not an absolute rule, but I'd say it tends in that direction.
While the politeness levels may be uncommon. スミス is not a nickname version of Smith. It‘s just accepting, that „th“ will get lost among japanese. Wouldn‘t it be good to make everyone feel comfortable giving up on „th“?
There's really not such a thing as a "Japanese-unfriendly" name, because whether your name is pronounceable with Japanese phonetics or not, the cultural expectation is that if you're speaking Japanese, you will introduce yourself as and answer to a name that conforms to the Japanese sound system. This expectaction makes EVERY name "Japanese-friendly" -- カルメリツ is exactly as pronounceable as カラー in Japanese, and it won't matter if your name is pronounced differently in another language, because neither you or the person you're speaking to will be using that pronunciation if you're communicating in Japanese.
Though, when you are converting your name to a Japanese pronunciation, make sure you do so based on the SOUND of your name, not how it's SPELLED. For example, "Carmellite" would be カーメライト, not カルメリツ.
Yeah, converting your name based on sound is very important. Mine's written "Geo", but pronounced closer to "Gyo", so I do ギョ, not ゲオ. That also happens to be much more natural for Japanese people to say :)
People often adjust their names so they fit the language they're speaking. I pretty much never use the native pronunciation of my (first) name when I speak English.
