Readouts that work autonomously on colorimetric principles were fabricated inside a microchannel by immobilizing dyes in a hydrogel matrix. Two pH-sensitive dyes (congo red and phenolphthalein) were studied. The response time of the readout varied between 20 s for phenolphthalein and 45 s for congo red dyes. The robustness of the dyed gels was tested at different flow rates (0.1, 0.25 and 0.5 mL min(-1)). Combinatorial readouts consisting of gels with the different dyes were fabricated and tested. Based on the limits of human perception, the fabrication methodology and the principles of functioning; general design rules for fabricating a readout device are discussed. The simple and low cost fabrication technique combined with autonomous functioning make the readout appropriate for portable microfluidic systems.