Nope, I'll be stripping my old orange marines, space wolf grey, dark angels green and making them all two tone!

Times like this I wish I'd stuck with no missile launchers for the tacticals... I'd probably have been able to come up with a full chapter then  

My DC is black with red crosses. And I'll make the assault squads both 4 and 6 strong so I can play codex or BA.

Good question.

-Do Blood Angels have to be red?

If you want to play as the actual Blood Angels original Chapter- Yes.
If you want to use the Blood Angels Epic Codex for your Marines- No.

-Do my red (i.e. Blood Angels) Marines have to use the Blood Angels list?

Background-wise, it's preferable, but you can do anything with background (the aforementioned mechanised BA army using Scions). Be aware that just because you can twist background how you want doesn't mean the opponent has to buy it.
If you don't care about background, no worries.

Rules-wise, as someone said- if the list is balanced, it's balanced regardless of what paint is on the models. My only tiny concern is confusion for the opponent- "That Death Company is actually a Devastator formation, those Baal Predators are Vindicators", that sort of thing.
If I see a Blood Angels army, my first instinct is it's a Blood Angels army- this isn't as bad in Epic, as most things and rules are universal, but in 40k armies have lots of wargear, unique special rules and units to confuse with.

Moral is never to copy an official Chapter (or Guard regiment, Eldar craftworld, etc), that tiny bit of creative thought to make your own colour scheme allows you to get away with murder when picking Epic Codex lists to use.

I have a terminator chaplain and a normal marine already on magnets so I can swap them in one unit, works well with some careful clearing out of the hole so they don't stand too tall.

As the marines all have chapter symbols on their sholder pads I can't do the different SC idea, but as its not using a 'famous' symbol I reckon I'm ok.

Thanks for everyone's point of view I'm going to shut this down now.