Strings are obtained using the Kolakoski substitution and the additional rule : start with 1 if previous string begins with 2, start with 2 if previous string begins with 1.

As n grows a(2n-1) converges toward A025142 (red as a word) and a(2n) converges toward A025143. Conjecture : a(n) is asymptotic to c*(3/2)^n for some c.