In our SAP system the users work with two different clients (110 and 200) in parallel. To avoid confusion between the two, we would like to assign different colors to these clients. Is this possible with GuiXT?

This is not possible. SAP GUI supports different colors for different SAP systems, but not for different clients within one system. With GuiXT, you can do the following for easy identification of the two clients: Create a GuiXT script "Esession.txt" (E= language key for English, F= French,...) that contains the statement

TitlePrefix "&V[_client]"

Each screen title will then start with the client number e.g. "110 Create Order". Alternatively, you can use

TitleSuffix "(&V[_client])"

which would result in the title "Create Order (110)".