When the color selection is disabled, it suggests that MRIcroGL believes this is a labelled atlas where continuous colors are not meaningful. You can click on the "Options" pulldown menu next to the color selection and choose 'Show Header". I assume the "Statistics" tab indicates the Intention is "Label". Perhaps you can determine why your statistical maps claim to be labels and create images without this intention. Alternatively, assuming your images are in .nii format (or after decompressing .nii.gz to .nii) you can use MRIcroGL to change the intention from "Labels" to "Not statistics" and choose "Save header" from the File menu to rewrite the header. Reloading the image should restore the color selection pull down.