색연필 (色鉛筆) is actually pronounced 생년필. An intervening ㄴ is inserted to replace the null initial between two morphemes 색 and 연필. This kind of insertion is most likely to happen when the following morpheme begins with a /j/ (이, 여, 야, 요, 유, 예, 얘) and the previous morpheme ends with any 받침, but there is no clear rule and many exceptions as well as variation between speakers.
鉛 having a “historical” nasal initial is strange since the character never had a nasal initial in ancient Chinese. Methinks it’s a process akin to nasalization of consonant codas before another nasal (a.k.a. it’s a “rule”), but I haven’t looked at the phenomenon in depth yet. Neither have I figured out the pattern to when ﾤ is inserted in the pronunciation of compound words since it’s unwritten. It’s unfortunate that Korean deviates from its otherwise morphophonological spelling in this respect.