Why does it flash green to red and not charging

All I can see is that your charger is flashing red. There wasn't any brand mentioned. Most battery chargers flash green when in charging mode, solid green when charging is complete and flashing red when there is an issue with the battery/batteries being charged. Rechargeable batteries don't last forever (usually 200-300 charges) and perhaps this is the case with yours. Also, if you are using imported batteries (China), that would be a major red flag!

If it changes from red to green and stays green while being charged, it is just a sign that it is being charged and there is nothing wrong.
If it flashes back and forth between red and green while charging, there might be something wrong with your charger's connection.

STATUS LIGHT will FLASH RED when
charging. Allow at least 30 minutes
for a full charge.
STATUS LIGHT will turn ORANGE,
and then turn off when charging is
complete.
STATUS LIGHT will FLASH GREEN
when the batteries in the CHARGING
BASE are LOW. Replace the
batteries.
STATUS LIGHT will FLASH RED &
GREEN when there’s a CONNECTION
ERROR. Reconnect the FAIRY.

according to manualslib type site for the wg540, solid red means charging for the wa3840 and wa3838. After 30 minutes if battery has not reached 10 volts (of 18), the red will begin flashing meaning defective.
Just green? Ready to go. Green flashing? Overheat.
Tangent; sometimes a discharged battery needs a little coaxing - have to get a little voltage into the battery for charger to try to charge. Different company, electric drill. I plead ignorance if this applies for the 3840 since they mention the half an hour time.

Ridgid charger lights read as follows: Solid red; charger has power. Flashing red; charger is evaluating the battery, checking if it's cool enough to charge and if it is in chargeable condition. If the battery is hot, i.e. just came off the tool after extended use, it won't charge the battery until it cools down. Flashing red & green; the battery is defective and the charger will not charge it. Solid red & flashing green; battery is charging. Solid green; battery is full.

According to the owners manual for these swimming pool lights, this is what each colored indicator means. Hope this helps.


Solid green
The battery pack is fully charged.
Slow flashing green
The battery pack is almost fully charged.
Slow flashing yellow
The battery pack is half charged.
Medium flashing yellow
The battery pack is less than half charged.
Fast flashing yellow
The battery pack has almost no charge.
Fast flashing red
A charging error has occurred and the light should be unplugged and then plugged back in. If the Fast Flashing Red persists after several attempts, the battery condition is questionable.



Also in the trouble shooting guide, it mentions this:


Problem: Red Indicator Light Is Flashing While Charging

Solution:
A red flashing indicator light when charging indicates a selftest
error condition. Unplug the product and plug it in again.
Try this several times; if the indicator light continues to flash
red, unplug the GlowBuoy and take it to an authorized service
depot.

What does indicator light mean?

• Steady red light: Battery is charging.
• Steady green light: Battery is fully charged.
• Flashing red light: Battery needs charging.
• Flashing green light: The music receiver is switched on.
• Flashing red/green light: The music receiver is available to be added to your phone.

Ok the lightning bolt is your power indicator: solid green-charging, Flashing green-fully charged, flashing red-low battery, flashing red three times the once green-bad battery. the "trash can" looking one is your hard drive activity indicator it will flash while your hard drive is reading and writing.

Have you activated phone yet? When phone is activated and in service it flashes green. It's hard to believe it would flash green without a SIM card, as there is no service...unless AT&T is providing emergency service only and you're getting a flashing green which means you're in range of their towers.

Then I'd say it's a reasonably sure thing that the green light is just burned out, The lights are supposed to be off at first, then red when there's a partial charge, then green when there's a full charge, then flashing between red and green when the "battery saver" kicks in. If you're seeing off, then red, then off, then flashing red, then that means everything in the charging circuit is working perfectly, there's just no green light.