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How big is your turtle? 2inch
How long have you had it? 4month

What is the water temperature? 20-27
Did you use a thermometer? no
Are you using a water heater? no
How much water is in there? until its shell
Are you using a water conditioner? no
Are you using any filtration? no

What is the basking temperature? 28
Is there a basking light? yes
Is there a basking platform that is easy to climb on? yes
What kind is it or what is it made out of? stone
Is there a UVB light? no

What have you been trying to feed it?
When was the last time your turtle ate?

How big is the tank/pond/enclosure?
Is the tank near a window? no
Is the tank in a room with a lot of activity? no

Have you read the Basic Care section? no
Have you searched the forums for similar situations? yes

my tortoise's shell is bright green, after i put it under basking place for few hour, it change colour to dark green..
why[b][/b]

What kind of enclosure is your turtle in? Do you have algae in your tank? It sounds like he has algae on his shell and when he was wet it's bright green, then he basks and it dries and becomes dark. If you don't have a uvb light and water is only up to the turtles shell you should probably read up on basic care.

How warm is the basking area? If it was too warm, it could have affected the color of the shell (kind of like a suntan/burn). When my RES basks outside in the sun for any length of time in the summer, his shell will darken.

the tank is clean and its shell change back to bright green....i think the water is too warm ..
is that bad or good ? the colour changing..

Can you post pictures of the color change?