Try rebooting your computer, if you haven't done so recently. In some cases a dark screen and rainbow textures (indoors/underground or at night) can be triggered eventually if the system is left on, until it is rebooted (it can also eventually go away on its own).

Try lowering the game resolution. One person reported that this was all it took to fix the issue in Divine Divinity.

In the Beyond Divinity install folder, edit the config.div file (in Notepad, or other text editor) and add/edit the line 'rgb 555' and change the number to something else, like 565. If the line isn't there, go into the graphics options and change the gamma, or something, and it should be added, or you can do so manually (just not as the last line in the file, as the game expects that line to be blank, and ignores it).

Maybe check out the Divine Divinity topic Graphical Issue, Indoor textures turning black?