Keep in mind that every computer monitor will show a slightly different colour.
If the description said 'yellow" and you got yellow, you have no Not As Described Case.
If the description said 'yellow' and you saw a mustard yellow item on your computer but got a canary yellow item, you have no Not As Described Case.
If the description said 'yellow' and you got a blue item, you have a Not As Described Case.
However, your first step is to contact the seller.
Ask about returning for refund.*
You will probably be told to return the item for a full refund.
Use a Confirmed Delivery service.
This can be expensive. If the seller is in China, you are looking at $40 plus for the return.
You will not be reimbursed by the seller or by eBay/Paypal for the return.
Consider donating the item or selling it on eBay if the cost of return is too high.
You may not make money on this, but it will be less expensive than return for refund.
*As a part of eBay's Buyer Protection, there are deadlines for returns and replacements. Money is simpler.