Sometimes they are solid yellow and sometimes solid red, often yellow with red streaks, or red with yellow streaks. If you want similar warm colors that bloom at the same time, then plant 'Beauty of Apeldoorn' tulips.
How long have you had them for? My Darwin ones lasted about 5 years . Then they were gone. Beautiful tulip border you have got by the way. Probably near to 400 tulips. How long it took you to plant the bulbs? Do you plant then deeper than recommended?