I have a few .gif images which are screenshots of a terminal window with a table of information; thus they are on a black background with white/yellow text, alpha-numeric.

I've tried the OCR file in the File Exchange (file 18169) but when it reads my color image and converts it to greyscale, the image is barely readable.

I know nothing about image processing; I use Matlab heavily for statistical analysis and it seems the only way I can get this data is via these screenshots, and I'm trying to avoid hand-entering these tables of data.

But this seems like something that is relatively easy to do - these are computer generated fonts on a high-contrast background. I'm hoping that there is a way to either more crisply convert it to a white background with black text image for the OCR file exchange program or an entirely separate program to do it. Is there?

Just make your own monochrome image.  Try taking one of the color
channels
redChannel = rgbImage(:,:,1);
greenChannel = rgbImage(:,:,2);
blueChannel = rgbImage(:,:,3);
Use whichever one gives you the best contrast.