I'm thinking of taking the red dragon prestige class after ten levels of bard and was wondering if I can still cast spells based of charisma, or if I'd need to rely on intelligence?

All your Bard spells will still be cast based on charisma. It should also be noted that unlike certain other prestige classes (Pale Master) you won't gain any additional spellcasting levels as you take levels in RDD- your spellcasting abilities will remain at whatever your highest level of Bard is.

According to the NWN Wiki, any levels gained in the Pale Master prestige class do NOT count towards your overall spellcaster level. Those simply afford You additional spell slots.

http://nwn.wikia.com/wiki/Pale_master

Pale master levels augment spell slots, but otherwise do not affect spellcasting. Thus, a level 10 sorcerer / 19 pale master has the same spells per day as a level 20 sorcerer, but has a caster level of only 10 and only knows as many spells as a level 10 sorcerer.

Yip, there's no prestige class that will continue to add to your caster levels unless you install PRC.

Bards are not great casters anyway, but make a perfect STR based melee warrior with RDD and FTR lvls.

B25/F5/RDD10 is still one of the best meleers around. (B8/F4/RDD8 at lvl 20 for maximum attacks per round.)

Ftr for heavy armour and (epic) weapo0n specialisation and 3 feats for a feat starved class.
Still spell, for casting in heavy armour during fights.
Curse song becasue it's unresistable.
weapon focus and improved crit for your chosen weapon.
Blind fight (cos all warriors need this)
Knockdown and Improved knockdown, the melee warriors mainstay feats.
Extend spell for (naked) pre fight buffing is also very nice, as is expertise.

14/16 INT to start, because you need those skillpoints, for tumble, discipline and perform at the very least.

14 CHA to start (cos RDD will give you the extra 2 you need for spell casting before you need them).

Heavy bard builds (16 (but better is 20)+) are also perfect for disarming characters with large weapons like greatswords as the AC from your song covers the loss of a shield. More ftr levels get you better BAB and extra feats to get improved disarm, and large weapon gives bonus to disarm chance.

have fun :)