A method for color image encryption was proposed. For encryption, a color image is separated into three components which are red, green and blue. The three grey images are transformed with Discrete Cosine Transform (DCT). Then these DCT spectrums are partially retained and merged into a single real value matrix, namely the composite spectrum. Thereafter, the matrix is encrypted by the diffractive-imaging schemes. The decryption is the inverse of the encryption. Since a special domain has been set in the composite spectrum, the algorithm for retrieving the composite spectrum has overcome the stagnation problem and converges quickly. Computer simulations show that a color image can be successfully encrypted into a single noise-like intensity pattern, and the decrypted image can be obtained with high quality.