How to reduce 3-COLOR to 42-COLOR? The requirement is that two adjacent vertices have different colors, and max. 42 colors.

I show that $ \text{42-COLOR} $ is in NP and then I must reduce it from $ \text{3-COLOR} $. Here it becomes complicated.

Is it similar to $ k\text{-COLOR} $ for any $k$?
For an instance of 3-COLOR, try to add a complete graph of size $k-3$, and add an edge between each new vertex and each old vertex. Now you can prove the new graph is $k$-colorable iff the old graph is 3-colorable.