Eye color change?

So, I'm going to try to keep this as short as possible, so to get straight to the point, my eyes change color constantly.
I was born with blue eyes and had them until I was 3. Then they went straight to dark brown. They remained that way for a few years, and then began to change color as I got older.
Now, they change on an almost everyday basis.
Some days they're dark brown, almost black. Other days they are bright green. They've been brown, gray, blue, Green, hazel and a yellow/amber color. Basically every color except purple.
I'm not crazy; Other people have noticed this too. My brother approached me the other day and asked if I was wearing fake contacts because my eyes were vivid green and he's used to them being darker (they usually are). My friends and parents have also taken notice of it.
I've researched stuff online but found nothing that matches what I've experienced. I have no eye pain or damage, minus a stigmatism in both eyes that I've had for about a year. But this started as a young child, not last year, so I don't believe the two are related besides the chance that the stigmatism may have somehow increased the color change.
I don't use any eyeproducts either; No contacts, no eye drops. I've never had laser eye surgery or any eye surgery or infection. NOTHING has happened concerning my eyes besides my stigmatism. No pink eye, nothing. No explanation. 
And don't accuse me of being crazy either because I know that I'm not. I thought that I might have been until other people started pointing it out. 
So my question for you is WHY THE HELL ARE MY EYES CONSTANTLY CHANGING COLOR??
PSA: Yes, some of these images are filtered. However, they don't change my eye color. My eyes were still that color on that day. If my eyes look really bright, it's because I was in natural sunlight more than likely.
Light/medium Brown: 
Dark brown:
Hazel Green:
Primarily Green:
Primarily Gray:
I know that these photos aren't the best but I don't take many of my eyes, and filter most of my photos without keeping the originals. I wish I had some plain ones of the changing colors that showed the significant changes, but now that I want some I'll make it a point to take some and possibly post them! Thanks for everyone's responses. 💜