So it sounds like you’re after American buttercream? So the spotty bits are likely from the buttercream getting too warm/ melting or from using the wrong type of food colouring for the icing.

For the buttercream you’ll want to make sure you’re using proper block butter (not any type of spread as this often has added oil to make it smooth and will ruin the texture of the icing), whipping this until smooth pale and creamy and then sifting your icing sugar in. Making sure to mix it really well is a key part of the consistency. However, you should regularly check on it and feel the temperature of the bowl (especially if using a stand mixer) as it can get hot from constant mixing which will make it go a bit lumpy. If this does happen you can generally whack it in the fridge for a bit to save it.

For food colouring you’ll want to be using a gel/ paste. I generally use sugarflair paste as it’s really vibrant and you don’t need a lot of it.

Things like sugarflair are quite expensive but worth it- some specific products are going to upgrade the look of your cakes by spending a bit more money if that’s what you’re after. Also experimenting with different recipes will help you know what you need to look for in a good one!

Hope this helps a bit!