Hi BordersMummy,
I'm similar to you - started with music at 7, ended up with Grade 8s at 15 and 17, was self-motivated to practice (non-musical family) and have continued music making through into my 30s (but never did it at university). So, I agree, early music is not a prerequisite to a love of music, or even being good at it.
However, I have 2 children aged 5 and 8. The 8 year old started piano (at his request) at 5. The first 18 months were a struggle - partly due to his age, and also his teacher. Since changing teachers things are easier, and he is eventually doing his Grade 1 this summer age 8.5. He then started the cello aged 7 (again at his request) - the background in music has helped him progress and he passed his Grade 1 10 months after starting.
My 5 year old started with a violin teacher aged 4yrs 10months - her teacher is excellent and experienced with starting young ones. She is self-motivated and passed her Grade 1 within 8 months.
Neither do Colorstrings but they do attend Kodaly and musicianship classes (which I believe are fairly similar concepts to Colorstrings). The enjoyment they gain out of these group sessions, together with the exposure to musical concepts helps them with their instruments. Also, at a young age they can be eager to learn and seeing my youngest thrive on it, I am glad I went with her wanting to start aged 4 (spurred on by her older brother playing) rather than following my gut instinct and waiting until she got older.
Maybe they will both get to age 13/14 and still be at a similar standard to those who started at aged 8/9, but the early start has meant that music has been part of their life and therefore practicing is normal (although can still be a struggle!!!).
I'd go with Colorstrings with DD2 - there is nothing to lose and perhaps a lot to gain!
Happy music making!