Cassandra and Colour
In TTS/RTA, when Cassandra wears red, it's representative of her loyalty to Corona and Princess Rapunzel. The most vivid red she wears is the sleeves of her guard uniform.
It's important that it's vivid, because this is when her loyalty to the kingdom itself is her priority. And it's the same uniform the other guards wear. It's also brighter than the red she wears as part of her 'action' outfit:
This is a much more brownish red, almost pinkish, which is interesting because pink is one of Rapunzel's preferred colours. This is significant because Cassandra tends to set aside her loyalties to the kingdom in favour of her loyalty to Rapunzel specifically, which is demonstrated the first time we see her wear this outfit - when she sneaks Rapunzel out of the kingdom for one night of freedom before her coronation.
Also interesting is that the red is worn mainly on her undershirt. From there we can assume that the red undershirt is actually touching her skin (at least on her arms and neck), and it's also presumably the last layer of clothing over any undergarments, meaning it's almost directly over her heart. Loyalty is something that, at this point, she is making her own decisions about.
But then, at the Great Tree, Cassandra's loyalty to Rapunzel is her undoing. Someone else summed it up elsewhere, but basically, Cassandra and Eugene both see Rapunzel's safety as their number one priority - but Rapunzel's number one priority is Eugene's safety. No one has Cassandra's safety as their number one priority, not even herself, and that's why she's the one to get injured. Her arm is badly burnt, even withered, so she puts on this outfit:
Two things are important to note here. First, the arm that was withered was Cassandra's right arm - her dominant arm, established in the next few episodes where she is forced to relearn how to use a sword. This is significant because it means her right arm is the one she uses to protect Corona and Rapunzel, and now it has been destroyed. She's slipped further away from that position of loyal bodyguard by another tiny amount.
The other thing to note is that she is still wearing red, but now, rather than a garment worn over her heart, it's a cape. The red is on full display, but now it's the furthest garment from both her skin and heart. She's shrouding herself in it now, because at this point she's trying to pretend that everything is still as it was and that she's still highly capable of doing the job she was given (first by insisting she's not as badly injured as she is, and then later when she begins to contemplate double crossing Rapunzel). But she's slipping further and further away, and her loyalty to Rapunzel is nothing more than a mask by the time they reach the Moonstone Chamber of the Dark Kingdom.
A third thing that's interesting to note is that the cape is a much... purplier shade of red than anything else Cassandra is worn. Where else have we seen a purplish red before?
In fact, knowing that the Great Tree - where Cassandra got the armour - was once Zhan Tiri's stronghold, for whom Gothel once worked, there's even the possibility that the cape is made from the same actual fabric as her dress.
There's more analysis to be done here regarding Cassandra and colours, but I'll put it in a reblog.
