In MATLAB, you can convert a grayscale matrix to an RGB matrix by replicating the grayscale values in each color channel (red, green, and blue).

To do this, use the repmat function to create an RGB matrix with the same number of rows and columns as the grayscale matrix, and with three color channels.

Here's an example of how to convert a grayscale matrix to an RGB matrix in MATLAB: 

% Create a grayscale matrix
gray_matrix = rand(100);

% Convert the grayscale matrix to an RGB matrix
rgb_matrix = repmat(gray_matrix, [1 1 3]);

In this example, the repmat function replicates the values in the gray_matrix in each color channel of the rgb_matrix. The resulting rgb_matrix has the same number of rows and columns as the gray_matrix, and three color channels.

You can also specify the size of the replication along each dimension using the size argument in the repmat function. In this example, [1 1 3] specifies that the gray_matrix should be replicated along the third dimension (the color channel dimension) to create a 100x100x3 RGB matrix.