Colour change normal or not

How do you tell the difference between velvet and colour change? My female betta is developing this yellow color at the base of her ventral fins (and its only visible under a flashlight) and I'm wondering if this is normal. She's also been showing stress stripes ever since I got her 2 weeks ago from petsmart. She's a typical blue veiltail betta (blue body, reddish fins) and the only fish in a 10 gal planted.

Hi Kaosu, the stripes are horizontal; kind of like in this pic:
http://www.getahugetank.com/images/betta-female.jpg
There are times when she's normal coloured (like when I first turn on the lights), but she turns stripey when I come close to the tank to feed/watch her- maybe she's afraid of me?? :S

The tank is filtered, 80F and gets 50% water changes per week. The Petsmart I got her at kept their bettas near the door-so she may have been exposed to drafts. She's still active and eating; her activity level varies though and she likes to spend a lot of time near the bottom in/around the plants and sometimes she clamps her tail. Seemed to be out and about more since I put in a baffle today (she was quite curious about that)- will see how she is tomorrow

I'll see if I can get a good pic where she isn't a fuzzy blur haha.

Well she's eating and active..so phew..I'm glad she's doesn't seem sick. It's been almost 4 weeks, so hoping her colours will brighten up with time. I feed her a small pinch of NLS weekdays, 3-4 bloodworms on Saturdays and fast on Sundays.

Anyway, I said I'd try to post pics so tada:

Yay She's a sweet fish- the boys I had in the past beg for food first thing in the morning, but she hovers mid-tank and only rushes over when she seems me coming. She's also active late into the night- perhaps its the weather (it's been really hot this week)..or maybe she's a night owl like me haha.