Choose the colour of your saree, veshti or function theme, and see the flower colours that sit well against it.
The most reliable rule is contrast. A garland that sits against the outfit rather than blending into it is the one that shows up in photographs — which is why white and cream garlands are the traditional choice against a red or maroon wedding saree.
If you want something softer, stay within neighbouring shades: pink with peach, marigold with gold, white with cream. These read as elegant rather than striking, and suit daytime functions and naming ceremonies.
Green foliage is the safe third colour in almost any combination. It gives the flowers something to sit against and keeps a bright pairing from looking flat under hall lighting.