How to understand the reduction from 3-Coloring problem to general $k$-Coloring problem? 3-Coloring problem can be proved NP-Complete making use of the reduction from 3SAT Graph Coloring (from 3SAT). As a consequence, 4-Coloring problem is NP-Complete using the reduction from 3-Coloring:


  Reduction from 3-Coloring instance: adding an extra vertex to the graph of 3-Coloring problem, and making it adjacent to all the original vertices.


Following the same reasoning, 5-Coloring, 6-Coloring, and even general $k$-Coloring problem can be proved NP-Complete easily. However, my problem comes out with the underlying mathematical induction:


  My Problem: What if the induction goes on to $n-1$-Coloring and $n$-Coloring problem, where $n$ is the number of vertices in the graph? I certainly know that $n$-Coloring problem can be solved trivially. So, is there something wrong with the reasoning? How to understand the reduction from 3-Coloring problem to the general $k$-Coloring one?
The $k$-coloring problem is usually defined only for constant $k$, so $n$-coloring doesn't make sense. For every constant $k \geq 3$, the reduction you mention works. By adding a superconstant number of vertices you can show, for example, that $(n/2+3)$-coloring is NP-complete.