I think you need to add "nonland". It's possible to have lands that are colored (zomg two Painter's Servants in play, set to red and blue. Or one and Dryad Arbor).
Or rephrase to "multicolored
spells".
Even ignoring the templating issue of not casting lands, is allowing that interaction really so terrible a thing? Most of the time that sounds like a three-card-combo equivalent to Exploration.