Remember the total number of chameleons never changes. There will always be 45.
The difference between each group of chameleons at the start is 2 and 4. Whenever two chameleons meet, each of those groups go down by one (-1), while the third goes up by two (+2). So the difference between any two groups can grow to 3 (the difference between the group that went up two and the group that went down one), or the difference can stay the same (the difference between the groups that both went down one). So the difference between groups can only change by multiples of three. In order for the two groups to get to disappear they have to have the same number of chameleons at some point. The difference starts out as 2 and 4 (not multiples of 3), so no changes by 3 will ever make them equal.