Yep. Just the regular blue Windex with Ammonia-D.
I haven't seen any impact on the paint color for Polly Scale at all and I've used it to thin a number of colors including RLM 04 Yellow and White - two colors that you might expect to be affected by a blue tint.
And there was no color change that I noticed with the Acryl paints either when thinned with Windex.
I believe that Model Master Acryl now has a new paint formula. I've noticed that the paint behaves a little bit differently so I've tended to use the Model Master Thinner with the Acryl paints. But for Polly Scale, it's good ol' Windex!