There's an interesting history behind why colonel is spelled and pronounced how it is...

https://www.deseret.com/1996/8/4/19258272/french-italian-roots-explain-why-colonel-has-an-r-sound

To investigate that question, we have to go back a little further into the word's history. The French word "coronel" is derived from the Italian word "colonnello." When the French borrowed the word, however, they found it difficult to pronounce. In an effort to ease the pronunciation problem, they changed the first "l" sound to an "r" sound. This is quite a common occurrence; when there are two "l" sounds or two "r" sounds near each other in a word, one of them is frequently omitted or changed to a different sound to eliminate a tricky pronunciation. Linguists call this type of alteration "dissimilation."

When English later adopted the word (in the 16th century), the French pronunciation was kept, but the letter "r" was changed back to an "l," making the term look more like the original Italian word and producing the conflict we continue to have between spelling and pronunciation.