In what year did different colours of Smarties (the chocolate with a sugar outer shell version) stop tasting different?

I don't believe that this answers your question directly, but it is related insight:

When they were first introduced, Smarties were known as ”Chocolate Beans” and were renamed Smarties a year later.

The original colours were dark brown, green, light brown, orange, pink, violet and yellow. Prior to 1958, the dark brown sweet had a plain chocolate centre, the light brown sweet had a coffee flavoured centre and the orange sweet had an orange flavoured centre. As a result of consumer research, this was then changed and milk chocolate was used for all the centres with only the orange sweet remaining flavoured.

https://www.smartiescollector.com/northerntrumpet/smarties_history.html

Sometime after 1958, according to the Smarties (Nestlé) website.
"The original colours were dark brown, green, light brown, orange, pink, violet and yellow. Prior to 1958, the dark brown sweet had a plain chocolate centre, the light brown sweet had a coffee flavoured centre and the orange sweet had an orange flavoured centre. As a result of consumer research, this was then changed and milk chocolate was used for all the centres with only the orange sweet remaining flavoured."
http://justfunfacts.com/interesting-facts-about-smarties/

Also, some people apparently have difficulty distinguishing the orange flavored orange colored Smartie from the rest.
http://candyaddict.com/blog/tag/smarties/

In any case, Smarties aficionados like to distinguish their candy of choice from the United States' M&Ms.
http://www.typetive.com/candyblog/item/head_to_head_smarties_vs_mms/

Can't find a reference for this, so understand if it gets deleted, but there are some varieties of Smarties that do taste different. For example, there was recently Smarties in Ice Cream flavours, e.g. chocolate, vanilla, strawberry and they tasted different.

My source is that I ate them! :)

I think guitargoddess's Smarties might be the US type which are quite different. The American things are more like the products of Swizzells Matlow in the UK - Refreshers, Love Hearts and so on. Nestlé (orig Rowntree) Smarties are more like M&Ms - and I couldn't envisage ice-cream flavoured ones.
https://www.smarties.com/our-story/