If you work on an estimate that your gold contains 17.5% silver
(3.5grams) and 7.5% (1.5grams) Copper, (bear in mind that this may not
be the case and there are many different recipes) You would need to
add enough gold and copper to the mix to reduce the percentage of
silver to about 5% minimum to turn the gold pink.
This means that you would have to increase the gold and copper
content proportionally so that you have 70 grams of pink gold.
70g of 18K pink
75% AU =52.5g
20% CU = 14g
5% AG = 3.5g
Is it worth adding that amount of gold just to save on wasting 20
grams? rather keep the yellow and mix a new batch of red as the true
red gold contains no silver at all and it is impossible to alloy a
mix that already contains silver.