We say goodbye to good old reliable 3430 in two weeks.   The boat is going to a new owner down on the Pamlico river and will see fair winds and great sailing.   From time to time some minor scratches had to be repaired in the yellow gelcoat hull, and I thought I would post some knowlege gained by trying to match that beautiful Flying Scot color.
At first glance you will say the boat is yellow, but if you tint white gelcoat with yellow you are going to be in for a shock.  That color is brilliant yellow, sort of a true bannana color.  When you dab it onto the scratch you will see how "circus" bright your new yellow is.  The color of the gelcoat must be tinted slightly with more than yellow.  It has taken a few years to get this, so please be patient.
Technically, I don't think that new activated Gelcoat resin actually bonds to 40 year old cured gelcoat, so a purist would grind all the way down to the fiberglass and build back up from there.  That being said, I have never had one of my scratch repairs "pop off", so, I'll leave the science to those that know.
I like to mix my Gelcoat on little plastic desert plates from the grocery store, as they are cheap as dirt, you will need a good stack of them.  I like to use those ultra cheap cut brushes to mix and apply the gelcoat.  They are cheap enough to just throw away.  Here we go:
pour out a small puddle of white waxed gelcoat onto a small plastic plate.  Use your cut brush to wipe the groove of the gelcoat container to avoid the glob that builds up there with each pour.
Next squeeze out  3/8" of yellow tint, and mix it super well with the cut brush.  You should have a bright yellow color.  If the color looks more like faded margarine instead of bannana, you will need to add a bit more tint.  Always go for less tint and then edge up on the color.  It is much harder to add more white.  If you do get to much yellow, I suggest you scrape half of what you have onto a new plastic plate and add your white to that.  Keep all you plates in front of you so you can see your progress.
When you think the shade of the color is about right, do the dangerous part. 
You have to add a microscopic amount of black tint to slightly dull the yellow color to match the hull.  The first time you do this, you will add way too much, and then just start over, like I said, you cannot undo too much tint.
Never try to squeeze black into your nice yellow mixture.  That is like 200 times too much!  Instead squeeze a tiny drop of black onto a new plate.  Then take the pointy end of a toothpick and drag what looks like nothing from the dob.  You will say that this tiny speck can't be right, and you'll get a bigger amount.  This will ruin the batch, and just start over now for the fun of it.
Drag the tiny toothpick with tiny black onto your yellow, and mix with the cut brush,  Dont work the toothpick back and forth to "get it all". You are alfter a really small amount.  You can always add a bit more!   Your mix will take on a faint greenish tint, that will blend in with all the yellow gelcoat.
Mix and mix, you have plenty of time as Gelcoat just sits there for hours and days without the activation chemical.  You are in no hurray.  Turn some music on, take a break.
Don't add the activator!   You are like Picasso.  You hold your thumb out and stare at your hull.  From a distance you try to guess if your duller yellow matches the hull.  Go ahead and dap a bit of your raw gelcoat mixture onto the hull., and step back.  How does it look?  First just try to guess if  your dab is lighter for darker than the hull.  Take off your glasses, so you just see a blurry view.  Decide if it is too dark or too light.  Next try to guess if the yellow is too "bright" or too "greyish green"  You are going to work back into there four color descriptions.
Fisrt start to adjust the yellow color.  If it is too bright, I would suggest you take half of your gelcoat to a new plate and add that much white.  You do this as it takes a lot of white to change a tint.  Mix that all up, and bush a tiny bit on the hull.  Now you compare first brush, second brush, and hull.  Does it look like the right yellow range?  You may need to do this 4 more times, no big deal.  Line up all you mixing plates so that you can see your progress, along with brush marks on the hull.  Don't worry, they all come off with a mere wipe of Acetone and paper towel.
When you guess that the depth of the yellow tint might be right, you may need to add a tiny bit more black to tone down the brightness of the yellow.  Be very careful, too much black, and by that I mean a tiny speck, can turn the artwork into sort of a pale greenish grey yellow.
You will know you are close when your brushed color sort of looks like the boat only wetter.  From my experience the cured color is about the same.  So don't tell youself that maybe it will cure "lighter" or something.  Before you apply your repair, wipe off all your experiments with gelcoat.
I'll not tell you how to prep your scratch, there are lots of videos for that.
I mix the drops of activater right there on the plate.  This makes it easy to have the drops all in different places.  Mix and mix and then mix some more.  You have plenty of time.
I just use the cut brush to apply the repair.  I "cheat" with this next idea.  You need some stiff clear plastic, like a report cover.  Avery Sliding Bar Clear Report Covers, Pack of 50 (47710) , or something like that lying around.  The idea is that you are going to gently cover your repair with this clear cover.  I like to cut my plastic about twice the size of my repair.  I apply masking tape along one edge and set it centered over the repair. Very gently "ooze" the repair throught the plastic and tape down the opposite side.  This is more art that science.  If you smoosh too much you will force the gelcoat out of the repair.  If you don't squish it some, there will be air gaps in the gelcoat.  If you do it right, the next day you can peel off the plastic, and there is very little sanding to do.  "Cheating"
I bet you guys know a much better way!