Does anybody knows How can I grid sea levels in GMT5 vis Windows?
You need to make more specific questions. Questions like this have basically no possible answer.
Have you looked at the Tutorial, namely this section?
Well, I use makecpt and -Cglobe. Sea levels have negative values. I would like to convert the Green colour to Blue colour in order to define the sea in my maps
How can I transform command grdimage in order to make it?
Please, again, you need to read the documentation (namely makecpt) and the tutorial. They explain things much better than one could here.
About color palettes