Red Light (Instead of Usual Green) on Power Cord when plugged in

Orange is charging, green is charged, and I seem to remember there's bright red when the connection is bad on the Powerbooks; it's probably the same on MBP.

Try moving the cable around or plugging it in upside-down. If not, you probably have a wiring fault somewhere in the adaptor. Visually check both plugs and where the cord connects to the white case.

Are you sure this is not an orange light? Orange means its charging the battery.

The bright red on the powerbooks means that the green light isnt on. See, in the color spectrum, Green and Red make Yellow. So when you see the "orange" charging light, its actually a dark orange (red) and green light on at the same time. My dad's AC charger for his TiBook's green light flickers sometimes and when its plugged in and charging sometimes it flickers "red."

I'm not sure about the magsafe chargers but it seems like its just 1 green LED and 1 orange LED that alternate.

Yes, however notice that OP says it isn't supplying any current to the MBP - he wrote:

"The computer initially reads that it is on ac power, but then switched to battery after the red light turns on."

Have you tried the usual? Resetting the PMU, zapping PRAM...? Removing the battery to see if it runs on AC only?

I think that red means it´s charging but green means fully charged, It turns red when I hook it up after using the battery and is now green (fullly charged).

when i have had problems with my powerbooks power... the charger glowed red instead of orange. indicating a problem with the power unit... thing...