Choose a picture

This is my first time to make a puzzle so it might be simply and easy.

You are trapped in a huge room. You found 5 different pictures on the wall.

Picture A- A red circle with a green background

Picture B- A purple oval with a yellow background

Picture C- A green square with a purple background

Picture D- A blue triangle with an orange background

Picture E- A yellow semicircle with a cyan background

Then, you saw a folded note in front of you. You unfold it and it says:

Dear ??,

Happy to be trap? I wish you are. You wanna escape? Here's your chance! Go flip the correct picture then you free. Go flip the wrong one then you will be a shark lunch. Hope you find it.

From: THE KILLER

You were panic as you didn't know where to start. However, you realized something is at the back of the note .

From: The same guy

Today, I am being generous and want to give some hints

Hint 1: 551a8b

Hint 2: One of the colour is primary for light.

Now, you stopped panicing, found the correct picture and escape.

Which is the correct picture? Answer with explainations. Spoilers will be out in 2 days. If no correct answer is given, I will answer the question