the evidence:

her room is purple (and green but mostly purple). her bed is purple with stars on it

the background during the hammer of justice fight changes to a purple color scheme with rain while gerson talks about the 6th chapter of lord of the hammer (which, for deltarune ch6, many speculate a dark world will take place in either the holiday mansion or the hospital). gerson's dialogue regarding this chapter of his book is as follows:

this is obviously gerson talking about himself in his own artistic process. and while many also speculate this to be reflective of toby's artistic process (which he may or may not express through the concept of gaster), i think it's applicable to dess herself. dess was clearly a creative, artistic person, with many hobbies and interests, who tried out many different sets of skills and sports and artforms:

gerson then goes on to center susie as part of his conversation with her:

and what does susie say about the knight to gerson?

it's no question that susie and dess are being set up as parallels to each other, perhaps even foils. susie is clearly a creative person as well, but because of her own circumstances, compared to dess, she never got the chance to really find ways to express herself.

and, pray tell, what color is susie?

that's right baby... she's PURPLE!!!