Abstract: In order to avoid the record and transmission of color image with complex ciphertext, we proposed to encrypt color images with a modified joint transform correlator by additional key multiplexing. First of all, a color image was separated into 3 components: red, green, and blue. Then the 3 grey images were encrypted with different additional keys to generate corresponding joint power spectra, and the encryption results were got, namely multiplexed joint power spectra by superposing the above mentioned joint power spectra. Furthermore, the decryption of the multiplexed joint power spectra with the correct additional key could give the corresponding primary grey images. Computer simulations show that the proposal can hide a color image into a single intensity pattern. Since recording intensity image is free from the reference wave, our method relaxes the environment and avoids interference equipment. In addition, compared with previous methods that encrypt image into complex ciphertext, the size of ciphertext of our approach is only half of it, thus the efficiency of the encryption is further improved.