When describing our Siamese I refer to the color of their eyes not just as blue. I describe them indicating where on the  blue spectrum they fall. To understand the color of a Siamese's eyes you first need to remember back to third grade science when you studied light. To be simplistic many colors make up light. The visible color spectrum ranges from red through purple. Remember the color wheel you made in art class? When we see colors we are seeing the waves of light that are being reflected back to us, all the other colors in the light are being absorbed by the item we are looking at. When all colors are reflected back an object is white. When all colors are absorbed the item is seen as black.  

All cats inherit genes that dictate how much melanin is in their eyes. The amount of melanin is what controls what colors are absorbed. (Melanin is also what our bodies use to protect us from the sun, our melanin increases as our skin tans.) In the Siamese the albinism mutation that causes color to collect on their points also causes only small traces of melanin in their eyes. The more melanin there is the lighter the cats eyes are. So a Siamese with very little melanin will have a deep blue eye.

 In reality the Siamese eye is made up of many shades of blue. This close up of  Maeve's eyes (un-retouched) shows these many colors. The overall effect from a distance is a single color. Siamese eyes can range from a very very pale blue that looks almost white to a deep navy. I have tried to create a visual example below. On the right are un-retouched photos of some of our Siamese. When looking at the photos at the top and comparing them to the photos below you can see the sky blue is more toward the aqua side of the blue spectrum and the lower photos eye colors are more toward the violet end. 

When photographing your Siamese the type of light you are working in will influence the color your cat's eyes will reflect back. Not all light bulbs are created equal. My best photos have been taken in strong sunlight. The sun has the full visible spectrum so you get good true color and the brightness of the light contracts the cats pupil so you have the largest canvas inside the eye. You will get red eye reflected back if the pupils are dilated.