the substance could be a pH indicator, soap is Alkaline, try exposing the substance to vinegar, if it is a pH indicator it should be a reversable process

The sticky substance is either tomato sap and/or honeydew left by aphids.Aphids suck up the sap of the tomato plant and excrete a clear sugary substance (honeydew). Ants and bees sometimes collect this honeydew.The honeydew is harmless to the plants but the aphids can damage the plant and spread plant disease. You can clean the aphids off by squishing them by hand or by spraying the plants with a strong stream of water. You can also get ladybugs (lady beattles) to eat them. Finally, you could use soapy water (dish soap or insecticidal soap). As to why the honeydew/sap changes color when you wash your hands, either you are also squishing aphids or some of the plant's green chlorophyll or sap is getting picked up / mixed in with the honeydew and reacting with the soap. "Because alkaline solutions mixed with tomato sap cause a colour change which stains hands, some nursery workers rub citric acid crystals..." -Broadbent The citric acid must change the pH of the alkaline soap, so the sap doesn't change color. Quinn SmithwickCambridge, MA, USA

The following answer has been selected and edited by New Scientist staffLeaves of plants in the family Solanaceae (including tobacco, tomato, potato and capsicum) all have minute hairs on their surface which exude drops of a sticky fluid.The function of this sticky substance is not altogether clear, but it could ward off attacks from aphids and other sap-sucking insects. The flavonoids and possibly other pigments in the fluid react with soap, which is alkaline, and change colour accordingly.David Whitehead, Cape Town, South Africa

The following answer has been selected and edited by New Scientist staffI remember being impressed long ago by the bright green colour that developed when washing my hands after helping my father tend his tomatoes. Some time later, as a botany student, I examined the tomato leaf epidermis under a stereo-microscope, with interesting results.The tomato epidermis carries two types of multicellular hair - long ones of several millimetres readily seen with the naked eye, and much shorter hairs with four glandular sacs like short sausages at the apex. These are filled with khaki-coloured contents and have very thin, fragile cell walls. I found that prodding one of these sacs with a needle released sticky contents that could be drawn out as a thin thread which set rigid within about 2 seconds, leaving a deposit on the needle.Later still, I found some photos of a mite wearing what can only be described as concrete boots. These were apparently made up of accumulated secretions from a tomato plant, collected as it rambled over the plant's surface. The deposit clearly encased the mite's legs, preventing it from hanging onto the leaf, thereby acting as a defence mechanism against this and other small, walking herbivores.Casual brushing against a tomato leaf will transfer only a little of the substance. This reserves the secretion for organisms that ramble among the hairs - or anything grasping the plant strongly enough to bend the longer hairs.The deposit's colour is lost against the skin of the average gardener, but it can certainly be detected by smell and, I suspect, by texture.Jim Kent, Minehead, Somerset, UK