Applying a keras model working with greyscale images to RGB images
Your model is not sufficiently complex to adequately classify the CIFAR 10 data set. CIFAR-10 is considerably more complex than the  Fashion-MNIST data set and therefore you need a more complex model.You can add more hidden layers to your model to achieve this. You should also add DROPOUT layers to prevent over fitting. Perhaps the easiest solution is to use transfer learning. I would recommend using the MobileNet CNN model if you want to try transfer learning. Documentation for that can be found here. Since CIFAR-10 has 50,000 sample images I do not think you will need data augmentation. First try a more complex model without augmentation and see what accuracy you achieve. If it is not adequate then use the keras ImageData Generator to provide data augmentation. Documentation for that is here.