Obviously i can't remember what exactly i did 5 years ago, but i think i went through my textures and made sure that all the textures that don't need extended bit depth, would be saved in 8 bits per channel and all the greyscale textures would be saved as single channel images. If that still doesn't solve low RAM issue, then the other most effective way to lower RAM consumption is to reduce texture resolution, just be aware that this potentially can come at the cost of lower final image quality. On the other hand, with modern Corona you may not need any of this, because it evolved since and introduced some memory saving features, like out of core textures.