Looking yellow & old due to weight loss at 21, is filler an option & where? (Photos)

First, shame on them for saying such hurtful things to you. You don't deserve that kind of criticism. Second, congratulations on your weight loss. That is extremely difficult to do and shows you have significant willpower.
The pictures make it difficult to assess your concerns adequately, but what I'd say is that if you are at a stable weight that you will maintain +/- a few pounds, then it would be okay to consider an intervention regarding volume changes to your face.
Regarding the color of your skin, see a licensed aesthetician for their opinion for options in your skin care regimen as this may be all that is necessary to bring out your inner beauty.

"I suggest reaching out to an expert for a formal consultation. I like to recommend my patients start a combination of facial rejuvenation treatments.

1) Skincare with products and microneedling/PRP
2) PDT Light Therapy a few times per month with growth factors
3) Laser treatments like Clear + Brilliant regularly and others (Fraxel, Moxi, Viva, CO2, Erbium, Halo) a few times per year
4) Sculptra injections a few times per year for collagen production with fillers like Juvederm, Restylane, Belotero, Radiesse, and/or Bellafill
5) Thermage/Ultherapy for yearly skin tightening with or without PDO or cone-based Instalift threads for lifting
6) Botox every 2 to 3 months to stop movement lines and prevent lines at rest

Best, Dr. Emer."

Thank you for your question and for providing photos. You may benefit from a combination of laser resurfacing treatments and dermal filler injections to address the concerns you’re referring to. Laser skin resurfacing can restore a radiant complexion by removing the outer layers of skin and revealing new, healthier-looking skin beneath. This treatment option can dramatically enhance the tone and texture of the skin and produce a refreshed, rejuvenated appearance. Hyaluronic acid dermal filler can also be administered under the eyes to fill in hollow areas and create a smooth transition between the lower eyelid and the upper cheek area. While injecting filler is less invasive than a surgical procedure, it still requires a great deal of skill and precision to achieve the best possible results. The use of a micro-cannula to inject the under-eye area is recommended to minimize bruising and swelling and ensure the smoothest outcome. I recommend scheduling an in-person consultation with an experienced, board-certified facial plastic surgeon or injector who can assess your concerns and determine the best treatment options for you.

It's difficult to give specific recommendations with the partial photos. Fillers can be used to help replace lost volume. Morpheus8 treatments combine microneedling and RF technology to improve overall skin tone and texture. We always recommend using medical grade skin care products such as Transform MD and wearing sunscreen every day. See a board certified plastic surgeon near you for a consultation to discuss your concerns and learn more about your options.

You’re still very young and you’re probably on the right track by being quite a bit thinner than before. Your body will rebound and the color will come back to normal. The hollow under eye area, however, may have been there because of your hereditary influence. The weight loss may have contributed in making it more obvious. I would not recommend using fillers in the area close to the eye as the skin is very thin. In my opinion, fat grafting would provide you with the best results which are extremely natural looking as well as permanent. You have to make sure however that the procedure is done by a plastic surgeon with lots of experience and fat grafting techniques. I hope this helps and good luck from New York City Plastic Surgery.