Can't record a clear video demo for the remote login tool

When I record a video demo using the remote login tool ---- putty. No matter what resolution I choose, I can't got the clear video. It seems the white characters are shown clearly while the red and blue ones are not.

The screenshot is attached below.

Does anybody know how to resolve this problem? I can get clear video if I choose the 'software simulation' mode instead of 'video demo', but here I have to use 'video demo' for my project.