Yes, any color other than white will get hot inside.  I have one of those
infrared hand-held temperature guns used by the A/C people and went around
shooting car hoods during the summer.  I thought maybe the silver wouldn't
be as bad in the heat but it was maybe the second best next to white.
Forget about black unless you like solar ovens.  Brother has one of those
dark metallic blue Matrix's and it get really hot one 100 degree days.

Fwiw, brother says the Prius has more interior room than his Matrix (he's a
bit over 6 feet tall) and was surprised how roomy  it was in back.  Seat
comfort was helped by adding thick sheepskin seat covers.  Only drawback to
sheepskins is they get pretty stinky in the heat (lot's of Fabreeze helps
for maybe a day or two).  They smell like a wet dog, if you know what I
mean, but they sure add comfort to the stock seat.

As to the color red, that was the last vehicle color I had that I got a
ticket in.  Was lucky enough to get two in one day!  Since then (almost two
decades later) I haven't gotten one speeding ticket in any vehicle - car or
fast (!) motorcycles that I've owned.  I just made sure they weren't red.
Go figger?

B~

"Seem" hotter?  If you expect it to be hotter then it will seem hotter.

Color actually makes very little difference to the maximum interior
temperature.  Parked here in Dallas, if I don't use my Sunshade the
interior temp of my silver Prius rapidly gets as high as 130'F.  But,
driving cow-orkers around on those same hot days those in the back seat
said they were comfortable.

Yes, as long as the A/C is running.  (Note that the vent in the back at
one end of the seat is for the battery.  This vent must never be covered
or blocked by a blanket, pillow, etc.)  

Never leave any child unattended inside any car.

Yes.

Undoubtedly some do, but extended warranties are insurance.  Like any
insurance, they are priced to make a profit for the seller, not for the
buyer.  If you can afford to "self insure" (that is, if you are able to
write a check for any repair that might be needed), that's always the
cheaper solution overall.

How much driving do you do? If you are a high-mileage driver it would
be wise to invest in extended coverage.