Only articles I have read, just state that amber ends indicate a 2 year old. The bird I killed in Texas threw me for a loop when I looked at it after I got home, had 1 1/8 and 1 3/16" spurs. All the 2 year olds I have seen have been pretty much all amber in the tips. My guess is 3 yr+ birds may have a few streaks if they have some melanin deficiency.

Left is a 2 yr old, middle is 3+, right is the texas bird