Trust me, you really are not missing much by not having played Red Sun. I was quite disappointed as it REALLY didn't feel like a Colony Wars game at all. Think War Thunder set in the Colony Wars Universe during the Vengeance timeline. While Vengeance had that boring token upgrade thing, at least it kept close to what made Colony Wars, Colony Wars.
The secret ending is exactly the same as the normal victory in Sol except it has about 15 seconds more at the end. If you got the secret ending, the victory in Sol plays twice with the 2nd one having the extra bit at the end. I can understand the Vengeance sequel though. After all, I did find it harsh that your LONE Eclipse would be sent with just THREE League Dreadnaughts to engage Tsunami. My first reaction, "Well ballz. At least these Dreadnaughts wil-...*Tsunami obliterates the 1st Dreadnaught with a single blast* ... This may take a while...". I had hope that they would eventually do a sequel based off the secret ending though.
I almost literally peed my pants when I heard they were making a sequel, as it was my thought that it was going to be based off the secret ending. I was sad to see that it was based off the common ending, but was still excited none the less. While I don't like that tech token upgrade system they implemented, I still found Vengeance rather enjoyable, even with the occasional "HHHHHEEEEEEEEEEEEEELLLLLLLLLL
LLLLLLLLLLLLLLP!!!" when you blasted a League fighter.

Then I found out about Red Sun. Once again, almost peed my pants as, once again, I thought it was going to be the sequel I was hoping for. Soon found out that this wasn't going to be so.

But I heard Vengeance being mentioned and thought that maybe there would be a sequel off of that one. Nope. War Thunder in the Colony Wars Universe set during the Vengeance timeline. I don't see it as a mistake to have purchased Red Sun (I still have all 3 games), as I will admit there were some fun bits in it. But to have the name Colony Wars ANYWHERE on it...

With the company that developed Colony Wars no more, there really doesn't seem to be any hope for it at all. What it may come down to is a group of folks redoing Colony Wars and/or developing a sequel based off the secret ending using the FS2 Engine, kind of like what that one group did with Wing Commander Saga.