There are a few options available:
- Convert grayscale images to colour images using standard OpenCV functions.
- Add a convolutional layer in front that has depth 3 as output. Proceed as per normal.
- Select one set of weights from the first layer of your existing network.
- Average the weights from the first layer of your existing network.
Each option has different trade-offs respectively:
- This is probably what would work well but you pay the price of extra computation, which may not be a lot in this case.
- You might essentially be learning filters that try to map to RGB, so this might be similar to 1.
- This is a hack, and may not work like how you would expect.
- Same as 3.