The information included in this page is intended to be a helpful guide to wearers of all types of contact lenses.
Whether you wear soft (Hydrophilic), RGP (Rigid Gas Permeable), hard, disposable, extended-wear, or daily-wear lenses, most
 of this information will apply to you. No advertisement or recommendation is contemplated by this information.

One issues in contact lens fitting continues to be the yellow lens. One of the most significant problems, particularly with high water absorption lenses, is protein. Protein can be denatured on the lens simply by tear fluid drying on the lens between blink. Repeated drying of the lens between blinks can create a deposit and potentially a yellow lens.
Also, a melanic type polymer derived from the polymerization of aromatic compounds in tears con deposit on hydrophilic (Soft) lenses and cause yellow to brown discoloration.
Another serious problem causing a yellow lens is patient non-compliance. Several studies reported indicate that inadequate maintenance of the lenses is a primary cause of contact lens failure. Almost 50% of all problems are attributed to failure to property clean and/or disinfect the lenses".
Fingerprints from handling lenses with unwashed hands, use of unapproved peroxide (e.g., OTC peroxide) may have additives such as colorants or other substances, and eye drops and medications can have a deleterious effect on contact lenses. Some eye drops containing phenylephrine, adrenaline, or berberine can cause yellow lens. Topical ephinphrine and vasoconstrictors such as tetrahydrozaline con also yellow lenses. Also, pyridium, tetracyline, phenolphethalein, rifamicin and nitrofurantoin are drugs that when taken internally and excreted into tears may stain contact lenses.
Environmental conditions may also contribute to yellow lens. Various salts or trivalent ions (e.g., calcium), pollutants, chemical vapors, cosmetic ingredients, water impurities, nicotine, oils and dirt from handling, may be present in the lens. Several references indicate that smokers often have a much higher incidence of pigmented lenses.
The nature and complexity of the yellow lens leads to a need to systematically review each case individually and avoid generalizations.
Answers to the following questions will be helpful in analyzing the cause and correct your mistakes.

What type of lens material are you wearing

Low, medium, or high water contents (The Low the water the better)

Are you on any medication?

What chemical composition?

Do you use eye drops?

What chemical composition?

Is it a first time problem, first time wear?

Have you been a successful wearer before now?

What care procedures had you been following?

Wash hands before handling lenses? (You should)
Brand of solutions used?
Do you reuse the solutions? Particularly for storage and disinfection. (Bad idea)
Regular use of enzyme? (A most)

Do you smoke?

Do you wear make up?

During this analysis many potential reasons will emerge. Strict adherence to the prescribed regimen, including enzymatic cleaning, as needed, will result in only limited instances and much less yellow lens.
Because this is a lens, solutions, patient compliance matrix of variables, the best way to prevent yellow lens is to use clinically proven lens and solutions combinations.
Experimenting with unproven regimens significantly increases the risk.