Gel-forming multi-component composition, preferably two-component composition, for forming a gel, the composition comprising or consisting of a component, comprising one or more cross-linkable, preferably cationically cross-linkable polymer(s), and a component, comprising one or more cross-linking agent(s) for cross-linking the cross-linkable polymers, wherein component and component comprise at least one dye, wherein the dye(s) in component is/are different from the dye(s) in component, wherein the at least one dye provides a transparent staining of the gel and optionally of component and/or component, wherein the at least one dye does not impair the cross-linking of the cross-linkable polymer(s) when contacting components and, wherein the at least one dye does not leak from the formed gel, and wherein the at least one dye comprises or consists of a stained molecule selected from the group consisting of dextrans, polyehtlyene glycol, starch, gellans, polyelectrolytes such as poly-I-lysine and other poly-amino acids such as poly(orni-thine), poly(arginine), poly(histidine); polypeptides and proteins such as gelatin, as well as polysaccharides such as chitosan, alginates such as sodium alginate, poly(glycolic acid), poly(lactic acid), poly-2-hydroxy butyrate, polycaprolactone, poly(lactic-co-glycolic) acid (PLGA), protamine sulfate, spermidine, albumin, carrageenan, furcellarane, pectin, xanthan, hyaluronic acid, sodium carboxymethyl cellulose, heparin, heparane sulfate, cellulose derivatives or sodium carboxymethyl cellulose, chondroitine sulfate, dermatan sulfate and dextran sulfate, for use in a method for removing undesired particles from a patient. the composition comprising or consisting of a component, comprising one or more cross-linkable, preferably cationically cross-linkable polymer(s), and a component, comprising one or more cross-linking agent(s) for cross-linking the cross-linkable polymers, a component, comprising one or more cross-linkable, preferably cationically cross-linkable polymer(s), and a component, comprising one or more cross-linking agent(s) for cross-linking the cross-linkable polymers, wherein component and component comprise at least one dye, wherein the dye(s) in component is/are different from the dye(s) in component, wherein the at least one dye provides a transparent staining of the gel and optionally of component and/or component, wherein the at least one dye does not impair the cross-linking of the cross-linkable polymer(s) when contacting components and, wherein the at least one dye does not leak from the formed gel, and wherein the at least one dye comprises or consists of a stained molecule selected from the group consisting of dextrans, polyehtlyene glycol, starch, gellans, polyelectrolytes such as poly-I-lysine and other poly-amino acids such as poly(orni-thine), poly(arginine), poly(histidine); polypeptides and proteins such as gelatin, as well as polysaccharides such as chitosan, alginates such as sodium alginate, poly(glycolic acid), poly(lactic acid), poly-2-hydroxy butyrate, polycaprolactone, poly(lactic-co-glycolic) acid (PLGA), protamine sulfate, spermidine, albumin, carrageenan, furcellarane, pectin, xanthan, hyaluronic acid, sodium carboxymethyl cellulose, heparin, heparane sulfate, cellulose derivatives or sodium carboxymethyl cellulose, chondroitine sulfate, dermatan sulfate and dextran sulfate, for use in a method for removing undesired particles from a patient.