Bluegeorge, first off he will not "start" to color up at 18 months, normally they are fully colored up at a year old and their colors can slightly change as they age. But at a year old you know exactly what they are. Also, I found that it also depends on the females lineage what your offspring look like. Many of us look at past offspring of the dame to see what kind of producer she is, then we pair her up with a male of the similar traits. And even then there are sometimes exceptions where you can have blue bar lineage from the female and male and still get an oddball red bar. So to answer your question, you cannot be 100% certain that he will end up just like the sire, but more than not they do.
-chris