Garter stitch would look the same on both sides but stockinette st also looks nice though different on each side. (You'll need a border on the stockinette st to keep it from curling.) You could consider making the central block in double knitting
which would be completely reversible and could be embedded in a garter st afghan. The double thickness would work well with the thickness of garter st.
When you change colors mid-row, you need to twist or overlap the yarns to prevent holes and to join the sections of color together. This video
shows intarsia which is one way of doing this.