jit.expr fills your matrix based on a formula. in this case you have a one plane matrix only: so if you have black its just the number "0", if you have pure white it is "255".

If you want to replace all the black pixels (= zeros), you could first transform the matrix in a 4 plane matrix (jit.matrix 4 char 200 100), then you could use f.e. jit.chromakey to replace black with another color, or you could use jit.op…