Netyjnd

I have a question as to whether or not legal documents signed in non standard pen colors (Anything other than blue or black) are valid.

I carry a purple pen around that use for everyday writing tasks, and when I was going to sign a document, someone told me that writing in purple is not valid on legal documents.

If the document does not specify that a certain pen color be used, is this true?

No

That is not correct. If the purple will not photocopy well, the other party might reasonably ask for a color that will. But a signature is normally only evidence of agreement to the provisions, and it is the agreement that is legally important. The color of the ink used does not change the agreement.

It is normal to expect a signature to be in a permanent ink. A signature in pencil or erasable ink might be legal, but the other party will not want to accept it, and it would be reasonable to comply.

someone told me that writing in purple is not valid on legal documents.

This is likely a misconception since most forms say use blue or black ink, but there is no law regulating a valid signature. In the US you can sign with an "X", a fingerprint, a yellow crayon if so inclined, a wax stamp, pencil, or even invisible ink* as long as it is meant to show valid acceptance. The objective is to demonstrate that you have agree to the terms in the agreement. Now the contract could stipulate blue or black in for valid acceptance of the agreement but this is on part of the offering party and must be stipulated prior to acceptance, not part of the law.

*Invisible ink may fail the communication requirement for contracts unless the other party is made aware of how to inspect the signature such as examination under UV light.

Also see a related answer for a related question.

Contracts, as a general rule, don`t even have to be written to be valid. But, some have to be because a law very often exists requiring this. The color ink used is normally irrelevant to its validity, unless the contract states otherwise or a statute (law). Courts usually have local rules requiring signatures on all documents be in either blue or black ink, but most banks will accept a signature on a check signed in any color.