The common strategy for using grayscale images in networks designed for color input, is to either replicate the images or to use only a single channel. Sometimes one of the channels is used for some sort of mask, but that depends on the actual project.
IM = uint8(randi([0 255],227,227,1)); % generate example image data
%option 1:
IM_replicated = repmat(IM,1,1,3);
size(IM_replicated)
%option 2:
IM_extended = cat(3,IM,uint8(zeros([size(IM) 2])));
size(IM_extended)
