So, tell me this! If red evolved into blue, then why do we still have red!?
Boom. Evolution disproved. You're welcome.
That's actually a question I get a lot, and it's a good one. On the face of it, your point does seem to have merit and explaining the thinking behind the correct answer is critical.
It's not that all red evolved into all blue but rather that the Red began to show a change in gene frequency at the population level. We must remember that evolution works at the population level, and in every population there will be random variation in allele types and frequencies. So, some Reds had a slight mutation.
Obviously there was some fitness advantage to the mutation. What that might be is often difficult to determine, but there was some else it would be selected against. Perhaps the Somewhat Less than Reds were able to exploit a food source more effectively, or had a resistance to a disease, or were better camouflaged; it could be anything. As time went on, and we're talking geologic time, some Reds maintained as Reds, and some Somewhat Less than Reds continued to show random variation and mutation in the alleles. Over time, they became a separate, or even several separate species from the Reds. At the far end, you have the Blues. At the origin, you have the Reds. And in between you show a clear progression of forms, some of which might have become distinct species, such as the Purple.
So, Red gave rise to the Purples and Blues, but not every Red evolved. We see a spectrum, which is another reason why I like the graphic.