Strickly speaking, no, you cannot disregard a placard or marking, unless there is another one, placed in accordance with an approved (by STC usually) installation, which changes/cancels it.
That said, what you say makes some sense, and is worth a follow through. Do the following:
Find the approval document, either on the Type Certificate Data Sheet (TCDS) for the aircraft, or the propeller, or an STC for the installation, and review them carefully. If there is a limitation, which would certainly include a yellow range marking, it will be stated there, or in a referenced document (but really should be on the certificate). The referenced document would generally be a flight manual supplement, which if it extists, you sure should have!
If there is no changed limitation, the yellow range marking still applies. If there no longer is a limitation, or it is different, there will be something which restates that situation.
If you have trouble finding the TCDS for the aircraft or the prop, tell me the exact prop model, and I'll find it for you. If it is an STC, it's not so easy, unless you have the number. You maintainer should be able to find that for you from the aircraft records.
Congratulations on ownership, enjoy the freedom!