When you see butterflies with different wing colors, you should conclude that there's a difference in their protein expression, so there's a mutation somewhere in the genome of the butterfly with the new color.
To answer the first question, if the desired color exists in nature, you should take the specific butterfly's caterpillars and raise them. If the desired butterfly color doesn't exist in nature than you should wait for the mutation to occur (which could take many years) or look for the gene in the butterfly which is responsible for the pigmentation of their wings.
For the second question, mutations in the gene of the coloration of the butterfly's wing could change the color, so what scientist do is to try to provoke different mutations on the caterpillar's gene until they have a butterfly with the desired color.
So everything is about molecular genetics, every difference between species is due to mutations.
