Well the main thing to remember is to ensure that the flowers are completely dry. any moisture inside the petals will turn the whole thing black and moldy within a few weeks - particularly in summer. If its a flower, best to use a waxy petaled variety like tuberose,gardenia, orchis species, roses, or any other thickly petaled variety- you can also go for dry petaled flowers like amaranthus sp., and zinias, gerberas, etc. I guess I should make a list as I have received many inquiries to resin dipped flowers lately- must be because Fire Mountain sells machine produces ones cheaply- or rather they are cheaply produces where they grow the flowers and marked up really outrageously by Fire Mountain and Rio Grande and other beader's supply stores on line and off!.
You can do your own resin dipping in clear-cast- available at most hobby or craft stores and remember to buy both the resin ( in a metal can)_ and the catalyst in a small plastic bottle- one won't work without the other. You should also assemble some wooden skewers, blotter paper 9 otr thick paper toweling, some acetone,and mixing cups that are disposable, a length of cord,twine or mono filament and clips from which to hang the dipped flowers and some nitrile gloves in case the gunk gets on your hands..many people react adversely to it); once you have all the equipment set up mix the resin according to directions, and CAREFULLY dip the flowers into the mixture-YOU WANT TO GO STRAIGHT DOWN AND DO NOT CREATE BUBBLES IN THE RESIN- so a bamboo skewer can be used to attach the flower at the calyx end ( stem end) and then let it rise naturally ( release the skewer and let it float to the top of your container that is at least as deep as the flower is wide and allow about 1 inch extra for each flower you want to dip. After fully curing - sometimes overnight, you may re dip if the coating is not thick enough on each piece. ensure that it is completely sealed.
Now for the colour retention part:
glycerine can be used to retain the shape and colour of botanicals however the botanicals must have a stem with which to soak up the glycerine and a bit of water( if you want autumn leaves for instance, you may add water soluble food colourant to the water to enhance the plant material's colour- just add a small amount and when the water is the colour you want add the glycerine 10 parts to 2 parts water..then allow the plant to soak up the glycerine.It will feel leathery to the touch and be preserved forever at that texture and colour, then dipping is easy as its cells are already "sealed" and the resin will cure slightly faster as no water has to evaporate or move through the resin ( in the form of O2) to completely cure in a humid area-as some people that live in the gulf south , or places in the Uk, and Europe that are not artificially air-conditioned do have humidity to deal with.If you live in the air conditioning or dehumidified environs, then it's less of an issue and curing is faster. Ther are some high priced resins that require UV light to cure - avoid them they aren't worht the expense unless you are to produce flowers commercially in which case I could recommend an entire equipment list but note that the cost will be about 15,000 dollars total to produce a commercially viable lot regularly once buyers are in place and the flowers sourced.I suspect you are writing for home use though and in that case, the "Clear-Cast brand " is sufficient for casting anything- just make sure you have ventilated work space a deep enough container for the material you are dipping as yu have to work rather fast, but carefully so as to avoid bubbles and the batch from setting up which is why I recommended only the depth of the flower's width plus an inch for the container- you can always trim a paper cup ( waxed paper) or plastic cup to the size needed and then have a mould ready to "embed something " with the unused portion that has been mixed up- anything can be embedded from artists tags, to pendants, to paper weights- you know the size of the materials you are to use.
Hope this helps, and please write again if you need clarification.
,best Regards,Dr.Ari Roark