can i trade in same iphone 8 plus but different color

Not after 14 days.

http://www.apple.com/legal/sales_policies/retail_us.html - Returns in the USA (check for your own country as some countries have different return periods): "For any undamaged product, simply return it with its included accessories and packaging along with the original receipt (or gift receipt) within 14 days of the date you receive the product, and we’ll exchange it or offer a refund based upon the original payment method."

You could try this:

Trade-Up program - https://www.apple.com/iphone/trade-up/ - "Just trade in your eligible smartphone, and then use the credit to lower the full cost of a new iPhone or to reduce the monthly payments with your carrier." Not available in all countries.

If you are past 14 days, you cannot just trade the phone for a new color. And iPhone 8 does not yet qualify for the Apple Trade Up program. If getting a different color is that important to you, sell the phone (you will surely take a loss) and buy the color you want.