As others have mentioned, kerosene is not the best solvent for the purposes of dissolving salts. Methylated spirits (denatured alcohol) is probably the best, and possibly rubbing alcohol, to give you the desired solubility you want. Caution - alcohols have a much lower flash point, are easier to ignite, are more flammable, harder to extinguish than kero, and burn with almost undetectable flame. They pose a significant flammability risk, especially if you spill some. You need to weigh these risks up for your performance, and seek some advice from your industry if you have not worked with these solvents before.
You can also consider pre-treating your implements by soaking them in the solution of desired chemical and allowing the solvent to evaporate (possibly for some days) and leave behind impregnated salt, which can then be burnt using other fuels. I have seen this done with pine cones soaked in aqueous solutions and allowed to dry out before being added to a campfire. You would still suffer from the competing flame colour and intensity of your chosen fuel. The colour of a kerosene flame will still probably dominate the subtle colours of your additives.
As for quantities; this will depend on the solubility of the salt in your solvent, and the general rule will be 'as much as dissolves', but will depend on the salt you are using. For instance, the solubility of sodium chloride (yellow flame) in ethanol (and therefore similar to methylated spirits) is less than 1g per litre, while lithium chloride (red flame) dissolves at something like 250g per litre. There are a couple of very good articles about the ins and outs of coloured flames for performance here:
Each different flame colour uses a different chemical which in turn has its own toxicity and undesirable residues that need to be considered for your own health (in preparing and performing) and also your audience (who may be breathing the stuff in). Be sure to read and understand the SDS of the chemicals you are using BEFORE you purchase and use them.
Fire colourants and additive kits are available for fire performers, and there are lots of very good sources on the internet for you to purchase and get advice for your performance: